People returning to England from abroad could face peak-time airport queues of up to six hours when travel measures are eased, a union has warned.
Fully jabbed UK citizens coming back from amber list countries will no longer need to quarantine from 19 July.
The Immigration Service Union said the "sheer number" of passengers arriving and a shortage of Border Force officers would slow down checks.
Transport Secretary Grant Shapps also admitted travel could be "disrupted".
But he added that airports, airlines and train and ferry companies were developing apps to cut queues.
On Thursday, the government announced that fully vaccinated UK residents returning to England from amber list travel destinations - including
popular holiday countries such as Spain, France, Italy and the US - would no longer have to quarantine from Monday 19 July.
But they will still have to present three documents in addition to passports:
- a passenger locator form, monitoring possible contact with people who have coronavirus
- proof of a recent negative Covid test
- proof of being fully vaccinated at least 14 days previously
Northern Ireland plans to adopt the same changes from 26 July, while both Scotland and Wales have said they will consider whether to do the same.
Airlines and travel agents have reported a surge in bookings for overseas holidays since England's new rules were announced.
But Lucy Moreton, the Immigration Service Union's professional officer, told BBC Radio 4's Today programme the government needed to recruit more Border Force workers to cope with the increased demand.
Ordinary passport checks took two to four minutes per person, she said, adding that those involving the Covid-related documents increased this to eight to 12 minutes.
"At the moment we are seeing peak-time queues somewhere in the one-to-two-hour mark, where we have got several aircraft that arrive at the same time," Ms Moreton added.
She told the BBC News website that larger passenger numbers would further slow checks and create airport queues "three times longer" than at present.
In response, Mr Shapps admitted travel would "be more disrupted than it was back in 2019", but the issue was "going to be on the check-in side rather than the border side" at Heathrow and elsewhere.
Airlines and train and ferry companies were "looking at digitising a lot of that", he said, adding: "That could help, but undeniably [travel] is more disrupted
BBC News.