Passengers arriving at London’s Heathrow Airport complained of waiting in a “Petri dish of mutations” while their documents were checked.
Some reports indicated that people were waiting for three hours at the border at Heathrow on Tuesday.
Home Affairs Select Committee chairwoman and Labour MP Yvette Cooper said queues and a failure to separate passengers were creating a “super-spreading risk”.
On Monday, Britain lifted the ban on international travel, with the green list allowing for quarantine-free trips between the UK and 12 countries and territories.
Travellers have also flown to amber-list destinations, though passengers arriving from those countries must quarantine for 10 days on their return.
Richard Bradley arrived at Heathrow on Monday from Vienna, Austria, after a work trip.
He spoke of his concern at having to queue with fellow passengers with limited social distancing.
“With the way we are being made to queue, l can’t help but feel we are in a bit of a Petri dish for mutations. Surely they can make this more efficient,” he said.
He told The National that although the situation had improved since a previous trip in February, it appeared that Gatwick Airport in West Sussex was "significantly better managed" than Heathrow.
Ms Cooper said it was “irresponsible” to have people queuing at the border. She questioned why electronic processing of Covid-19 documentation was not put in place before leisure travel was resumed.
“If you have people waiting for long periods of time in a not brilliantly ventilated arrivals hall, often standing very close to each other, well that’s a super-spreading risk … especially if you have people arriving from red list countries alongside people arriving from green-list countries,” she told the BBC.
“If they don’t have those things in place at a time when we’re all desperately trying to keep the progress moving forwards, there’s a real risk that we’ll end up just going backwards again.”
Prime Minister Boris Johnson said on Tuesday no one should be travelling to an amber-list destination without good reason, despite it being legal to land in the UK from those countries.
“It is not somewhere you should be going on holiday – let me be very clear about that,” he said.
“If people do go to an amber list country, if they absolutely have to, [it should be] for some pressing family or urgent business reason. So I think it’s important for people to understand what an amber country is.”