Stansted Airport’s operators have called on the government to explain how it assigns countries to the red, amber and green travel lists.
Travel agents and a trade union representing airport workers have also raised concerns.
Charlie Cornish, the CEO of Manchester Airports Group (MAG), said yesterday (June 24) that there is "no transparency" from the government on how Covid-safe international travel lists are drawn up.
He said this makes it impossible to plan for a restart to international travel this summer.
The UK government updated its international travel lists yesterday, with 16 destinations added to the green travel list including The Balearic Islands, Madeira and Barbados.
Travellers entering England from the 27 green list countries do not have to quarantine upon arrival.
The government also announced restriction-free travel for people with two vaccine doses later this summer.
Mr Cornish said: "The government has failed to publish the data it is using to inform its decisions, which makes it harder to prepare for any future changes to the lists.
"With no transparency from government, we simply cannot understand how Ministers are taking these decisions, which are fundamental to the future of a sector that supports more than one million jobs."
Stansted Airport is the largest single-site employer in East Anglia.
The airport suffered a 95 percent drop in footfall year-on-year between March 2020 and March 2021.
Of the travel list update, Mr Cornish said: "This is a long overdue step towards a restart for international travel, giving customers more options when looking to book a holiday this summer.
"However, serious questions remain as to why these destinations weren’t opened-up sooner when there is clear evidence to suggest this could have been done weeks ago."
MAG and Ryanair launched a legal challenge on June 17, calling for a Judicial Review of the red, amber and green lists, and how they are drawn up.
On June 23, MAG staged a social media protest, posing 272 photographs bearing the flight codes of each departure from the same date on 2019.
Just 71 flights departed Stansted that day.
Some aviation workers also gathered outside the Palace of Westminster, London, to protest against the government's coronavirus restrictions on international travel.
They were joined by travel agents from Flitch Travel, Dunmow, based just over six miles from Stansted Airport.
Rebbeca Kingston, Flitch Travel co-owner, said: "We have worked harder than ever before, and not received any pay for it."
She added: "The changes marked a step in the right direction.
"We just need the list to grow more and more.
"There is no need to keep us prisoner in this country.
"The biggest question is: When are they going to allow people who have been vaccinated to come and go?"
Rebbeca said changing the rules often leaves customers confused.
She said: "My message to customers is: Just ask. That's what we're here for."
The next steps
Trade union GMB has called on the government to introduce industry-specific measures for the international travel industry.
Nadine Houghton, GMB National Officer, said: "The aviation industry has been ravaged by this pandemic.
“There is a desperate need for a sector specific furlough arrangement, especially as a full summer season now looking impossible for aviation.”
The furlough scheme is due to come to an end this autumn.
The government currently pays up to 80 percent of a worker's salary up to £2,500 per month, but the amount subsidised by the government will reduce to 70 percent from July 1. Employers will need to make up the final 10 percent.
Nadine said this left aviation workers facing economic uncertainty.
She said: "GMB members working in airlines and ground handling companies face continuing uncertainty over their jobs, and continue to face pay cuts."
Nadine added: "The addition of countries to green lists is welcome - as is the possible easing of restrictions to amber countries for those with a double jab."
International leisure travel resumed on May 17.
The next review is expected around July 15.
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