British holidaymakers could be kept waiting inside their aircraft to prevent overcrowding at border control when they return home.
The aviation industry is expecting long delays as England eases restrictions on foreign travel from next Monday.
People fully vaccinated against Covid-19 will be able to travel to most countries without having to isolate on their return.
But with checks still required at the border, UK officials have drawn up contingency plans to prevent airport terminals from being overwhelmed.
One of these plans would require passengers to wait in the plane on the tarmac rather than disembarking as soon as it lands.
The contingency plans, seen by The Times, would kick in if border queues are taking longer than four hours to be processed.
Lucy Moreton, an officer from the Immigration Service Union, said the system was not equipped to manage the new level of demand.
“There is no way that the border can maintain that level of checks as the number of travellers increases,” she said. “From the number of bookings we’ve seen already, we’ll easily see three, four-hour queues when people start returning.”
The UK government expects airlines to check people’s vaccination status before allowing them to board a plane.
However, people arriving in Britain will still have to hand in passenger locator forms and go through passport and immigration checks.
The International Air Transport Association gave a warning that Covid checks could leave passengers facing an eight-hour delay in departures and arrivals.
Average waiting times had already doubled to three hours and were likely to increase as passenger numbers rise, Iata said.
Foreign holidays were banned until May 17 when the government launched its traffic light system to resume travel.
It opened up a handful of green list countries where holidaymakers could travel without having to isolate on their return.
But ministers announced last week that holidays to amber list countries would no longer come with quarantine for fully vaccinated people.
The new policy will take effect on July 19, when Prime Minister Boris Johnson plans to lift all restrictions on social contact in England.
The move to ease travel will make travel easier to popular holiday destinations such as France, Spain and Greece.
However, the plan is limited to travellers who were vaccinated through the UK’s National Health Service, excluding people who got their shot abroad.
John Holland-Kaye, the chief executive of London’s Heathrow Airport, called on the government to extend the policy to vaccinated US and EU citizens.
“These changes will be critical for exporters who are losing out to EU rivals and families who have been separated from loved ones,” he said.
Meanwhile, new figures showed Heathrow passenger numbers in June were almost 90 per cent down on pre-pandemic levels.
About 957,000 people used the airport last month, compared to 7.2 million in June 2019.
UK Transport Secretary Grant Shapps said widening the scheme to expats and others was "something we are very actively working on at the moment".
He said he expected that "in the next couple of weeks I'll be able to come forward and say more about other locations in the world".