Image source, Win McNamee/Getty Images

Sondland reiterates that all his efforts were "in good faith and fully transparent", and reported up the State Department chain of command, all the way to Secretary of State Mike Pompeo.

"Everyone was in the loop," says Sondland. "It was no secret."

"State Department leadership expressed total support for our efforts," Sondland adds.

"Our efforts were reported and approved. Not once do I recall encountering objection."

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Sondland addresses a mobile phone call he made to Trump from a restaurant in Kyiv. This call only came to light last week in other testimony.

"Other witnesses have recently shared their recollection of overhearing this call," says Sondland.

"For the most part, I have no reason to doubt their accounts," he says.

"It is true that the President speaks loudly at times. It is also true that we discussed A$AP Rocky. It is true that the President likes to use colourful language."

Unrelated to the impeachment inquiry is the matter of US rapper A$AP Rocky, who had been detained for assault in Sweden at the time.

US official David Holmes, who was in the restaurant, has testified Sondland told Trump on the phone that Zelensky "loves your ass".

"So, he's gonna do the investigation?" Trump reportedly said.

Sondland, Holmes said, replied that Zelensky is "gonna do it" and would do "anything you ask him to".

Image source, Getty Images

Image source, Reuters

Sondland says he told Ukrainians that the release of US aid would "likely not occur" until Ukraine made a public statement on the investigations demanded by Rudy Giuliani and Trump.

"This security aid was critical to Ukraine's defence and should not have been delayed," Sondland says, adding that he pushed for a statement on investigations "to break the logjam".

"I really regret that the Ukrainians were placed in that predicament," he says. "But I do not regret doing what I could to try to break the logjam and to solve the problem."

There's a lot here about who said what to whom and when. And that's important. But there's a wider picture too which we shouldn't lose sight of.

This is an inquiry which could lead to impeachment.

Impeachment is the first part - the charges - of a two-stage political process by which Congress can remove a president from office.

If the House of Representatives votes to pass articles of impeachment, the Senate is forced to hold a trial.

A Senate vote requires a two-thirds majority to convict - unlikely in this case, given that Mr Trump's party controls the chamber.

Only two US presidents in history - Bill Clinton and Andrew Johnson - have been impeached but neither was convicted and removed.

President Nixon resigned before Congress took a vote on impeaching him.

Image source, MANDEL NGAN/AFP via Getty Images

Sondland says he first made contact with Giuliani in August, when the president's lawyer "emphasised that the President wanted a public statement from President Zelensky committing Ukraine to look into corruption issues".

"Mr Giuliani specifically mentioned the 2016 election (including the DNC server) and Burisma as two topics of importance to the President."

"Burisma" refers to the energy company where ex-Vice-President Joe Biden's son worked. Trump has claimed Biden had a Ukrainian prosecutor fired in order to shield his son from a possible corruption investigation.

The "DNC server" refers to the Democratic National Committee, and the "CrowdStrike" conspiracy theory that claims the Ukrainians tried to influence the 2016 election to help Trump's opponent.

Trump mentioned these unsubstantiated claims in his July call with President Zelensky.

Sondland describes a text message exchange between himself, former Ukraine special envoy Kurt Volker and current Ukraine ambassador Bill Taylor, who have already testified in the impeachment probe.

Taylor writes to Sondland that the Ukrainians were told by Trump's personal lawyer, Rudy Giuliani, that the White House meeting would not happen.

"Good grief," Sondland says Volker responded.

Sondland says these messages are evidence that Giuliani had been communicating with Ukraine "without our knowledge".

Giuliani's involvement persisted, Sondland says.

"Even as late as 24 September, Secretary Pompeo was directing Kurt Volker to speak with Rudy Giuliani," Sondland says, referring to Secretary of State Mike Pompeo.

Pompeo, America's top diplomat, has largely sidestepped the impeachment investigation so far.

But Sondland's statement places him squarely in the middle.

OK, so one name is coming up a lot already.

Many people will remember him as the New York City mayor who was widely praised for the way he led the city through one of the darkest moments in its history, the 9/11 attacks.

But he's done a lot in his career and has come into the orbit of Donald Trump several times.

Here's a rundown.

Image source, Reuters

Sondland dismisses suggestions he was involved in "irregular or rogue diplomacy" as "absolutely false", a response to charges that Sondland used back-channels with Giuliani to circumvent typical diplomatic processes.

But Sondland says explicitly that the requests from Trump's personal lawyer "were a quid pro quo".

In order to secure a White House meeting with President Trump, Ukrainian President Zelensky was to issue a statement announcing investigations into the 2016 US elections and Burisma, the gas company that employed former Vice-President Joe Biden's son, Hunter.

"Mr Giuliani was expressing the desires of the President of the United States," Sondland says.

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Sondland - a hotelier and Trump financial donor before being appointed ambassador - has denied that he "muscled" his way into the Ukraine relationship, as some reports have claimed.

He says that after attending the inauguration of Zelensky on May 20, 2019, the US delegation was "excited that Ukraine might, after years of lip service, finally get serious about addressing its well-known corruption problems".

Trump, he said, was sceptical after they told him that he should hold a call with Zelensky and later invite him to the White House Oval Office.

Trump, he said, "even mentioned that Ukraine tried to take him down in the last election". That refers to a conspiracy theory which claims, without evidence, that Ukraine sought to help Trump's Democratic opponent in the 2016 election.

Trump then directed the US team to "talk to Rudy" referring to the president's personal lawyer, Rudy Giuliani.

Let's rewind for a second to explain why Gordon Sondland is such a central figure here.

He recently filed a three-page addition to his original testimony to congressional impeachment investigators after the testimonies of other witnesses refreshed his memory.

So what did it reveal?

The BBC's North America editor Jon Sopel explains the significance of the ambassador's testimony in 60 seconds.

On 26 July this year - a day after the Trump-Zelensky call - Sondland says he met President Zelensky in the Ukrainian capital Kyiv, and also talked with senior Zelensky aide Andriy Yermak.

"While I do not recall the specifics of our conversation, I believe the issue of investigations was probably a part of the agenda," says Sondland.

Later that day, he says, he spoke on the phone with Trump for about five minutes.

"I remember I was at a restaurant in Kyiv, and I have no reason to doubt that this conversation included the subject of investigations."

Secretary of State Mike Pompeo has so far today refused to address Sondland's claim that he kept him "in the loop" regarding his diplomatic efforts with Ukraine.

Pompeo has been drawn further into the impeachment hearing today, with Sondland directly linking him to the US request for Ukrainian investigations into Burisma and the CrowdStrike conspiracy theory.

Sondland addresses the 25 July call between Trump and Ukraine's President Zelensky, when the US president asked his counterpart to investigate the Bidens. This conversation sparked a whistleblower complaint and the impeachment inquiry.

The ambassador says he first read the transcript of the phone call in September, when it was released by the White House.

"Looking back, I find it very odd that neither I, nor Ambassador Taylor, nor Ambassador Volker ever received a detailed read-out of that call with the Biden references," Sondland says.

The wealthy hotelier, who donated $1m to Trump’s inauguration, describes "concerns" from US officials who heard the call.

But he says "no one shared any concerns about the call with me at the time, when it would have been very helpful to know".

Image source, Getty Images

Sondland says he, cabinet member Rick Perry and special envoy Kurt Volker were asked to work with Trump's personal lawyer, Rudy Giuliani, "at the express direction of the president".

"We did not want to work with Mr Giuliani," Sondland says. "Simply put, we played the hand we were dealt," adding that working on US-Ukrainian relations was conditioned on working with Giuliani.

As BBC's Anthony Zurcher says - the testimony draws a direct line between Giuliani and Trump.

Still, Sondland says that he did not believe Giuliani's involvement was "improper".

Top committee Republican Devin Nunes rubbishes the Democratic-led impeachment inquiry.

He says Democrats have repeatedly "floated the idea of malfeasance" by Trump. Nunes mocks media reports that claimed Trump was a secret Russian agent.

After special counsel Robert Mueller's inquiry - which investigated Russian meddling in the 2016 election - Democrats "dropped the issue and moved on to their next asinine theory", says Nunes.

"Democrats have gathered zero Republican support," he points out, calling the investigation "an impeachment crusade".

Sondland will say he and his fellow diplomats "weren't happy with the President’s directive to talk with Rudy [on Ukraine matters]. We did not want to involve Mr Giuliani".

They chose to work with Giuliani, "not because we liked it, but because it was the only constructive path open to us".

"Trump tried to take advantage of a vulnerable ally", Democratic Chairman Adam Schiff says in his opening statement, adding that now it is up to Congress to decide how to respond.

"We have not received a single document from the State Department," adds Schiff.

Secretary of State Mike Pompeo and Trump made a "concerted across-the-board effort to obstruct this investigation and this impeachment inquiry", he says.

"I will say this. They do so at their own peril."

He adds that President Richard Nixon, who resigned while facing impeachment, was also accused of obstructing justice by refusing to comply with a subpoena for documents.

Image source, Getty Images

Trump is accused of breaking the law by pressuring Ukraine's leader to dig up damaging information on a political rival.

In July, he urged his Ukrainian counterpart to investigate one of the frontrunners to take him on in next year's presidential election. This matters because it is illegal to ask foreign entities for help in winning a US election.

An impeachment inquiry that could see the president eventually removed from office is under way.

But there is a fierce debate about whether Trump broke the law or committed an impeachable offence - he himself says he has done nothing wrong.

Want more detail? Read the full story here.

Ahead of today's testimony, President Trump went back on Twitter to criticise the "hoax" inquiry.