Tui has announced it has joined Virgin Atlantic and British Airways' parent company International Consolidated Airlines Group 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 Holdings 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.
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?"
Tanzer issued a "heartfelt plea for political change" in relation to restrictions on international travel.
Noting that outbound travel is influenced by several government departments, he said the sector is not content to be "a political orphan".
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."
Tanzer said Aviation Minister Robert Courts pulled out from a planned speech at the conference due to "a diary clash".
Flintham said: "It is incredibly disappointing that the aviation minister didn't come to speak with us all today.
"There is no doubt the government needs to hear what we have to say as an industry, and this once again feels like a sign that they're not." ■