Tui has announced it is joining Virgin Atlantic and British Airways in supporting legal action against the UK government’s coronavirus travel restrictions.

The UK’s largest tour operator said the three firms have become interested parties in a challenge launched by Ryanair and Manchester Airports Group last week.

The legal bid is an attempt to get the Government to be more transparent in relation to how it determines which countries are on the green, amber and red lists under the traffic light system for international travel.

The risk-based system with red, amber and green ratings for different countries, determines the quarantine and coronavirus testing requirements people face when returning to the UK.

There are currently no major viable tourist destinations on the quarantine-free green list.

Speaking at the Travel Matters conference organised by industry association Abta, Tui managing director Andrew Flintham said: “At the time of the last country review, many destinations such as Malta, the Greek islands and the Balearics had much lower rates (of infection) than the UK.

“It was inexplicable as to why these were not added and instead Portugal was moved straight from green to amber, without the slightest sign of stopping at the much-vaunted green watchlist.

“We must understand the criteria we are all working towards so we can pre-empt when countries may move into different categories and help our customers with that challenge, and we must understand how the framework is being applied.”

Abta chief executive Mark Tanzer told the conference that the organisation is also looking at whether legal action is “an avenue that we can pursue”.

He said: “The hurdle for suing the Government is high but we think at least the Government needs to say, did it measure the impact on the travel sector of its own policies, and if it did, did it then decide that the sector nonetheless wasn’t worthy of support?”

Mr Tanzer issued a “heartfelt plea for political change” in relation to restrictions on international travel.

He added: “Clear accountability for the welfare of the outbound travel sector needs to be given to a designated minister.

“Our economic contribution is weighty. More money is spent in the UK by British citizens prior to travelling abroad than is spent by international visitors, and the job creation or destruction potential is huge.

“I say to Government: put aside any misguided prejudices against outbound travel. We are ready to work together, show us that you are.”

Last week, Ryanair boss Michael O’Leary has said the “stop, go, stop, go approach to travel is bonkers”.

He told Sky News: “It’s typical of Boris Johnson’s Government, just making this stuff up as they go along.

“There is no green list.

“What we keep calling for in the travel industry is, now that we have 80% of the adult population of Britain vaccinated, why can’t those people go on holidays to Portugal and Spain without restrictions? They’re already vaccinated.”

Tory MP Huw Merriman, chairman of the Transport Select Committee, told the summit that the UK has been “sluggish” in reopening travel.

“We have led the world when it has come to international aviation travel standards, yet when it’s come to this pandemic we have not done so,” he said.

“We have not played an international lead where we really could’ve done so in terms of the rules.”