What's happening with foreign travel?

International travel for non-essential reasons like going on holiday has been legal since 17 May, but with so few countries on the green list, the destinations where travellers can go without quarantining are few in number.

While the government has always said that international travel would be limited, the travel industry had not foreseen that the list would be as limited as it is, particularly now that Portugal has been moved to the amber list.

So what could get travel going again and what are the roadblocks?

Coronavirus variants

While the government is concerned about importing new cases into the UK, the biggest concern is that tourists could bring in a vaccine-resistant variant that would render vaccination efforts useless and endanger public health.

The difficulty is working out when a variant - or in Portugal's case - a mutation, is significant enough to change a country's traffic light status.

Is it when it is classified as being "of concern", "of interest" or just when it's identified in the first place?

"The move from variant of interest, to variant of concern, has never been properly identified as the parameters to look at," Dr Bharat Pankhania, senior clinical lecturer at University of Exeter's Medical School tells the BBC.

"They mostly all start off as variants of interest and become variants of concern when displaying activities and properties of being more infectious, severe disease causing and bypassing the current vaccines."

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But where that line is drawn is important, particularly if the identification of a variant or mutation is enough to move a country from one level to another.

Some in the industry think the government is becoming too risk-averse.

"There's talk of a new variant and everything shuts down," says budget airline Jet2's boss Steve Heapy. "Is that how it's going to be going forward?"

"If it is, then they're trying to defeat mother nature - virus do have variants all the time."

Tim Alderslade, the chief executive of industry body Airlines UK, says the sector isn't calling a reopening of travel everywhere.

"But if the government is going to have this kind of risk appetite...then it's going to be really difficult to see how we can have a summer season," he says.

A spokesman for the Department for Transport said: "We have always been clear that protecting public health is our priority and that if decisive action was needed, it would be swiftly taken."

Will islands reopen?

The industry had hoped that while mainland Greece and Spain might not yet be on the green list, at least some of the Spanish and Greek islands would be.

The travel industry had hoped that some of the Greek Islands would be on the government's green list for foreign travel

The island policy was used last year and Transport secretary Grant Shapps said in April that it was something his department wanted to consider.

But there are difficulties with looking at individual islands, partly because of their size.

"Healthcare facilities are limited and they can provide what they can and no more," says Dr Pankhania.

"There could come a time when a British holidaymaker is told, wait - we have Spanish people here who are sick and they could take priority, leaving holidaymakers stuck."

There are also worries that while coronavirus testing is showing infection rates for island residents, it isn't including the test results for tourists, who might still spread it amongst one another.

However, others argue that including smaller regions on the green list would helpful keep travel flowing where the risk is low.

How is the list decided?

This is a sore point with the travel industry, which says it doesn't understand what tips a country between the different colours.

The government is worried about tourists and visitors bringing in a variant of the coronavirus that could prove to be resistant to vaccines

While the government has laid out criteria like vaccination rates, virus levels, variants and the quality of genomic sequencing offered by a country, it hasn't set out exactly what level of each leads to a move up or down the list.

"We can't plan what we're doing,' says Mr Heapy. "We've either got to assume all the destinations will be green, none of the destinations will be green or use a lottery machine and try to work it out in the absence of a structured methodology."

Ultimately, this isn't just a decision about other countries' data.

While ministers are given recommendations by the Joint Biosecurity Centre about other countries' positions, they also weigh up other issues too, like the UK's health situation, or the practicalities of putting a country on the red, amber or green list. This is still a political decision.

What are other countries doing?

There's nothing in the government's guidelines about international business travel, or taking what other nations are doing into account when deciding whether to put a country on the green list.

Yesterday, transatlantic airline bosses came together to put pressure on UK and US politicians to reopen flights between the UK and USA for business and leisure travel.

They want to see US President Joe Biden and Boris Johnson commit to announcing a travel corridor at this weekend's G7 meeting in Cornwall.

There is no doubt that time is of the essence for airlines, whose finances have been stripped by a year with minimal passengers.

Delta Airline boss Ed Bastion said it was rare to see all his aviation competitors come together, but that they were "speaking in one voice".

He warned that as Europe begins to open up to US travellers, the UK could be left behind, missing out on lucrative tourist income.

The European Commission has announced that its EU Digital Covid-19 Passport will begin on 1 July, and may also be issued to non-European travellers, including visitors from the UK and the US.

However, European nations are already beginning to welcome travellers.

Mr Lundgren says people want to go on holiday as soon as they can

Spain, which is on the amber list, started allowing British travellers to enter without the need for a Covid test or vaccination certificate from Monday.

So you could visit Spain, but you'd still have to quarantine on your return.

"It just beggars belief," Travel Trade Gazette editor Sophie Griffiths tells the BBC. "Especially as countries are opening up and Britain is closing down. People are just confused and frustrated about why that's happening."

EasyJet boss Johan Lundgren agrees: "This decision has cut Britain off from Europe and from the world for that matter - it's utterly confusing for the British people."

It is becoming a fine and delicate dance to find the equilibrium between the demands of travel bosses and governments.

"I can see where [travel bosses] are coming from," says Dr Pankhania.

"But from an infection control point of view we are finely balanced now. We are nearly there - it would be better to be completely certain before opening the gates to all, [as] once opened, it is difficult to close them again."