Holidaymakers are being warned they will face "unavoidable" 10-hour delays at UK airports for the rest of the year.
Mammoth queues are already being caused at the London airport due to Covid-related checks as the UK battles to prevent mutant strains coming into the country.
And this is before overseas travel gets the formal go-ahead from May 17 and the government releases its "green list" of safe countries.
The Immigration Services Union (ISU), which represents Britain's border immigration and customs staff, is urging Downing Street to make drastic changes for when the ban on non-essential travel is lifted.
The union said delays of up to seven hours were seen when borders opened last summer, and fears they will be even worse this time and last for months.
Stringent checks include all arrivals being spoken to by immigration officers to ensure they haven't arrived through a "red list" country before they are allowed to pass through the e-gates.
Any issues with paperwork can often take up to half an hour to put right, leading to swelling queues of disgruntled passengers.
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Lucy Moreton from the ISU told The i newspaper: "We saw delays for seven or eight hours last summer, and with all the additional checks then we could see people waiting as long as 10 hours.
"There's no way around the delays at the border because Border Force officers will have to check the Covid status of all arrivals and that takes around 15 minutes per person.
"So, people from all over the world will be mixing inside for a long time."
While British Airline Pilots Association (BALPA) General Secretary Brian Strutton told MailOnline : "Sorting border queues is absolutely in the Government's control, and they must do it now.
"As I explained at the recent Transport Select Committee hearing, the prospect of hours upon hours of queuing at airports is just another barrier that will put the public off travelling.
"We need passengers to feel confident, safe and secure when looking to travel this summer because without their bookings, the travel industry which is so important to the UK economy, is in serious trouble."
Simon McNamara, from the International Air Transport Association, told the i he is "not optimistic" the Government has a working plan for when borders open later this month.
Heathrow, British Airways and Jet2 have already called on Home Secretary Priti Patel to increase border guard numbers to help alleviate the "inhumane and completely avoidable" queues already being seen.
Earlier this week, Prime Minister Boris Johnson said the UK must take a cautious approach to re-opening non-essential foreign travel to prevent an "influx of disease".
Tomorrow Downing Street is set to announce its 'green list' of countries where Brits will not have to isolate on return for 14 days.
