Holidaymakers could face 14-hour waits at Dover under new border controls requiring people to provide fingerprints and photographs.
MPs were told that tourists heading to Europe could be forced to join mammoth queues around the Port of Dover when the EU Entry-Exit System launches in October. Ashford Borough Council told Parliament's European Scrutiny Committee that 14-hour queues are a "reasonable worst case" scenario if the scheme is implemented as planned.
Delays at the port could also create long tailbacks, potentially blocking access to the Eurotunnel in Folkestone. Eurostar also warned that there could be queues of more than an hour in terminals at peak times.
High Speed 1, which runs UK high-speed rail services, said that the decision not to enable online pre-registration would "put enormous pressure on infrastructure at St Pancras International".
Sir Bill Cash, Chairman of the European Scrutiny Committee, said: "Queues of more than 14 hours, vehicles backed up along major roads, businesses starved of footfall: this evidence paints an alarming picture of the possible risks surrounding the Entry-Exit System's implementation.
Michelle Mone's husband gifted Tories 'over £171k' as Covid PPE row rumbles on"Clearly, this policy could have a very serious impact, not only for tourists and travel operators but also for local businesses. I implore decision makers on both sides of the Channel to take note of this evidence. The scheme is due to be implemented in October this year; the clock is ticking, and these issues must be urgently addressed."
But Downing Street tried to downplay the risks of delays. A No 10 spokeswoman said: "On some of those scenarios, rightly we're not going to get into hypotheticals about delays.... but there are tried and tested contingency plans in place and a lot of work is managed by the Kent resilience forum so we will make sure we're prepared and our focus remains on ensuring the scheme does not unnecessarily impact local communities or resort in delays for travellers.
"We are working very closely as you would expect with the French, with all the port authorities to ensure that travellers and passengers do not experience unnecessarily delays, that work continues."
Downing Street also sought to reassure Brits there would be no shortage of red roses on Valentine's Day due to post-Brexit border checks. Labour MP Daniel Zeichner raised concerns in the Commons over the difficulties for importing flowers.
But a No10 spokeswoman said: "I'm sure people will be receiving bouquets of flowers on Valentine's Day this year."