According to the travel union, queues in airports could reach six hours after the travel restrictions ease on July 19.
The increased time in the airport is due to both COVID-19 checks and Brexit.
When Britons arrive in European countries, they can no longer use their e-gates at border control but have to go through a manual passport control check.
Professional Officer at Immigration Services Union, Lucy Moreton, said: “There still will be queues associated with EU exit, with staff perhaps not being as available.
“Particularly if self-isolation figures surge in the way people are suggesting they might do before 19 August.
“But while we retain the requirement to check… [Covid] documentation for every arriving passenger… these queues are going to happen”.
Airport queues are approximately two hours now but after the travel restrictions ease for vaccinated Britons, the time will likely go up drastically.
On the UK side, passengers have to go through COVID-19 related checks performed both by UK border staff and the airline before travelling.
When arriving at their destination, Britons will find long queues for different checks depending on the country’s COVID-19 test requirements and passport control.
Immigration procedures are now slower for UK tourists as the border force has to manually check the documentation and Britons are no longer allowed to use the e-gates for quicker entry.
Transport Secretary Grant Shapps commented on the matter.
“The place to expect queues is the airport you are coming from.
At the moment, UK travellers arriving from a green list destination must present proof of a negative COVID-19 test result, a passenger locator form and pay for two PCR tests.
Fully vaccinated Britons will be able to do the same from July 19, when quarantine will no longer be required when returning from those countries.
Those coming from amber countries will need proof that they have received the COVID-19 vaccine at least 14 days before travelling.
Holidaymakers can use the NHS app or an NHS letter when entering a European country this summer.