One man who knows how it feels to be waiting for a family member to return home in this situation is Richard Ratcliffe, husband of Nazanin Zaghari-Ratcliffe - the British-Iranian national who was held in an Iranian prison for six years.

He's been speaking to the BBC about the mix of emotions relatives can expect.

"You're on tenterhooks and you're so nervous. Until [the plane] has left Iranian airspace, I'm sure they're not quite letting themselves dream, to believe that it's really going to happen.

"For the families, it'll be hard when to let go and start to feel euphoria; for the prisoners, it takes a long time to feel safe again... it takes a while to really be ready to feel it."

Zaghari-Ratcliffe was arbitrarily detained in 2016, initially on spying charges which she strongly denied.

She was widely seen as a hostage used by the Iranians to pressure the UK government to pay a long-standing debt, and was released once a diplomatic settlement was reached.

Caroline Hawley
Diplomatic correspondent

The deal struck between Britain and Iran last year saw the UK repay a long-standing military debt to Tehran – and Morad Tahbaz’s fate was part of the negotiations.

The UK Foreign Office believed, at the time, that it had secured his release into house arrest in Tehran. In the end, however, he was allowed out of Tehran’s notorious Evin jail for just 48 hours.

Iran clearly hoped it could use his American passport to their advantage too, in negotiations with the US.

Morad Tahbaz was put back behind bars until last month when he was again released into house arrest, as the final details of this complex agreement were worked out.

Now, he and his wife are in the air, with just hours to go until they are reunited with the rest of their family.

Relief for prisoners, but scepticism lingerspublished at 14:30 British Summer Time 18 September 2023

14:30 BST 18 September 2023

Lyse Doucet
Chief International Correspondent, reporting from the airport in Doha

The enormous relief that some prisoners are finally coming home is tempered by the knowledge that more foreigners may be seized in future. There are still other dual nationals behind bars in Tehran.

Meanwhile, Qatar is hoping this rare moment of co-operation will help to jump start progress on other long-standing disagreements and disputes - including the 2015 nuclear deal regarded as all but dead after former US President Donald Trump pulled out of it in 2018.

Others are more sceptical.

Prisoner swap won't lead to wider reset in relationspublished at 14:23 British Summer Time 18 September 2023

14:23 BST 18 September 2023

Caroline Hawley
Diplomatic correspondent

The ordeal of dual nationals held hostage by Iran for leverage has always posed a tricky dilemma for their governments. You want your citizens free, but by making deals with Iran, are you only encouraging further detentions?

A diplomat in Tehran says the detention of dual nationals has become a “business model” for Iran’s Islamic Revolutionary Guard Corps.

This complex and controversial agreement - months in the making - is a breakthrough for the US and Iranian governments. And a huge moment for the families.

But Iran and the US remain bitter adversaries - at odds over Iran’s steadily advancing nuclear programme, over its appalling human rights record, and over a US military build-up in the Gulf to protect shipping lanes.

Don’t expect this prisoner swap to lead to a wider reset in Iran’s relations with the US and the West.

Iran's Press TV says two of the five Iranians to be released by the US have landed in Doha.

There is no independent confirmation as yet of this, but officials have said the other three are not returning to Iran.

Sanam Vakil, director of the Middle East and North Africa programme at the Chatham House think tank, has told the BBC she thinks the use of the word "hostage" to describe the prisoners is appropriate.

"The Islamic Republic is a hostage taker... This is a business that [the Iranian government] has been in for well over two decades... They also take their citizens hostage through unfair judicial practices, disrespect for human rights - this is indeed a hostage-taking regime in Tehran."

Vakil tells the BBC that from Washington's viewpoint, the deal shows the Biden administration has stuck to its word.

Lyse Doucet
Chief International Correspondent

The Qatari jet has taken off with the five prisoners and two relatives accompanied by the Qatari ambassador, an official briefed on the matter tells the BBC.

Lyse Doucet
Chief International Correspondent

At long last, this rare moment of co-operation is unfolding.

We've just been told that the five American prisoners have arrived at Tehran airport; it is about a two-hour flight until they land at Doha International Airport.

It is a deeply controversial deal, highly complex, but also very compassionate.

US President Joe Biden has vowed to bring all Americans home, including these five Americans who also hold Iranian passports.

The US describes them as "wrongfully detained", but they're widely regarded as hostages, used by Iran as bargaining chips.

Iran can't access the $6bn (£4.8bn) of released Iranian funds, which has arrived here in Qatar this morning, directly - only third parties can use it for food and medicine.

This is the end of a deeply painful, personal ordeal.

The BBC understands that five prisoners have arrived at Tehran airport and are now with the Qatari ambassador to Iran, according to a source who has been briefed on the situation.

We've heard from a few different analysts this morning, as we await confirmation that the expected prison swap is fully under way.

Iranian-born Professor Mehran Kamrava, who teaches at Georgetown University in Doha, told the BBC that both the US and Iran will be reasonably pleased with the deal.

“I think there's a little bit of a win for both sides,” Kamrava said. “For Biden, heading into the election, he's bringing Americans home. For Iran, there's the release of Iranians in prison in the United States. But it's that six billion dollars that's a big win.”

Jason Brodsky from United Against Nuclear Iran and Middle East Institute's Iran Programme told the BBC he did not think it was wise for the US to link the release of funds to the agreement.

"There is precedent for the United States to engage in deals to release American hostages in Iran without the release of financial assets and I think that's the framework that we should be pursuing", he said.

Image source, Getty

In recent days we've seen news of the prisoner swap elicit mixed reactions from US lawmakers, with many Republicans harshly condemning the Biden administration over the deal.

One Republican - Utah senator and former presidential hopeful Mitt Romney - said the deal was a "horrible idea" that would lead to "more kidnappings".

"That's why you don't negotiate with terrorists," he was quoted as saying by The Hill. "That's why you don't negotiate with kidnappers".

Similar statements have come in from several of the current candidates for the Republican presidential nomination, including Tim Scott, Ron DeSantis and Donald Trump, who alleged - without any evidence - that Joe Biden was receiving "kickbacks" from the deal.

While the White House and State Department have both defended the swap, reaction from Democrats has been considerably more muted.

Speaking to reporters last week, New Jersey Democrat Bob Menendez, the chairman of the Senate Foreign Relations Committee, acknowledged that there were "concerns" about the deal, although he did not criticise it.

Senator Mark Warner, the chair of the Senate intelligence committee, said in an interview with CBS on Sunday that he hadn't been briefed and that he hoped the Biden administration would provide more details on the "guardrails" put in place to govern how Iran used the money released as part of the deal.

Once the swap is complete, we are very likely to get more reaction from both sides of the US political spectrum - stay with us.

Lyse Doucet
Chief International Correspondent

We're at the old international airport here in Doha and it will soon be the scene of an extraordinary political moment - a rare moment of cooperation between Iran and the United States.

This is where we are expecting five American prisoners to touch down, where they will be met on the tarmac by American and Qatari officials before finally going home to the US.

Families of those from the US will be hoping that they will be home soon.

This will be the end of a deeply painful, personal ordeal for these Americans.

But it is a moment laced with profound trauma and also with guilt, because they know they will be leaving friends behind who are still behind Iranian bars.

The five American prisoners are reportedly being transported to a Qatari aircraft to leave Iran, according to a source speaking to Reuters.

We'll give you confirmation as we get it.

Image source, Reuters

While we wait for updates on the transfer of the American citizens to Qatar, let's take a look at what we heard earlier today from the Iranian foreign ministry.

Spokesperson Nasser Kanaan confirmed that based on the information that he has, the five citizens of Iran in US prisons will be released today and five US citizens who have been imprisoned in Iran would be "delivered to the American side".

Speaking at a press conference about the swap, he also talked about the transfer of the funds, which was a precondition of the deal. He said: “We hope to see the complete repossession of assets by the Islamic Republic of Iran today."

He added that he expected the issues to be "completed according to the agreement".

Image source, Reuters

All of the five American citizens expected to be released have all been held in Tehran's notorious Evin Prison.

Known for housing Iranian dissidents and prominent political prisoners, it is nicknamed "Evin University" because of the number of intellectuals and students there.

Evin Prison was built in the early 1970s, under the country's then leader, Shah Mohammad Reza Pahlavi, and run by the Shah's security and intelligence service, SAVAK.

The prison has long been criticised by Western rights groups. Human Rights Watch has accused authorities at the prison of using threats of torture and of indefinite imprisonment, as well as lengthy interrogations and denial of medical care for detainees. Many detainees have been held in solitary confinement.

A group of hackers calling themselves Edalat-e Ali (Ali's Justice) posted videos in August 2021 of leaked surveillance footage from Evin prison showing guards beating or mistreating inmates.

Last October people were injured in a blaze at the jail in Tehran.

Among those who have been held there is the British-Iranian national Nazanin Zaghari-Ratcliffe, who was arbitrarily detained in 2016 and held in prison for six years, initially on spying charges which she strongly denied.

She was widely seen as a hostage used by the Iranians to pressure the UK government to pay a long-standing debt, and was released once a diplomatic settlement was reached.

This deal comes after indirect talks mediated by Qatar, that began in February 2022.

There were at least nine rounds of difficult discussions in Doha with the American and Iranian delegations staying in separate hotels, according to a source briefed on the negotiations.

Senior Qatari officials also shuttled between Tehran and Washington.

The prisoner deal first emerged on 10 August after the US agreed to the transfer of $6bn (£4.8bn) in Iranian funds from South Korean to Qatari accounts.

Image source, Getty Images

As we've been reporting, the prisoners set to be released by Iran are said to include Morad Tahbaz, a wildlife conservationist who holds Iranian, American and British citizenship.

His daughter Roxanne Tahbaz staged a protest with Amnesty International in April last year outside the Foreign, Commonwealth and Development Office, calling on the UK government to do more to bring him home.

The FCO has previously said it was working closely with the US to urge the Iranian authorities to permanently release Tahbaz and allow him to leave.

Image source, Getty Images

Image source, Wana / Reuters

Speaking last week to American broadcaster NBC, Iranian president Ebrahim Raisi said that the funds that form part of this deal should be returned as the money is rightfully Iranian.

Raisi said the money was "...unjustly frozen by the United States... we believe this American action was very unjust from the beginning".

As our chief international correspondent Lyse Doucet has been reporting, the funds - revenues from Iranian oil sales to South Korea - had not been frozen assets, but were unavailable to Iran because of various complications including currency conversion.

US Secretary of State Antony Blinken told Congress last week that the money would be "of limited benefit" to Iran, given it could only be used for humanitarian purposes.

But President Raisi said Iran would choose how best to spend the money.

"This money belongs to the Islamic Republic of Iran," he said. "Naturally, we will decide to spend it wherever we need it. Humanitarian means whatever the Iranian people need. This will be decided and determined by the Iranian government."

The five Iranian prisoners who are expected to be released have been named by Iranian officials as:

• Kaveh Lotfolah Afrasiabi was detained near his home in Boston in 2021 on charges of "acting and conspiring to act as an unregistered agent" of the Iranian government.
• Mehrdad Moein Ansari and Amin Hasanzadeh were both said to have links to Iranian security forces
• Reza Sarhangpour-Kafrani and Kambiz Attar Kashani are both accused of violating US sanctions against TehranOut of these five, two Iranians are expected to remain in the United States and one of the Iranians will fly to a third country to join his family. Two others will return home, Iran's foreign ministry spokesman Nasser Kanaani told Reuters.

Lyse Doucet
Chief International Correspondent

This is the scene on the tarmac of the Doha International Airport, where we and other members of the international media are poised, waiting to hear confirmation that the plane with American Iranian prisoners has taken off from Tehran.