Airlines threaten legal action as all arrivals to Britain must self-isolate for 14 days to curb the spread of coronavirus starting on Monday.
Michael O'Leary, group chief executive of low-cost airline Ryanair, said "millions of jobs" could be lost as a result of the quarantine rule, which applies to arrivals, including British citizens, from all countries except Ireland.
Conservative Prime Minister Boris Johnson said the quarantine requirement, initially until June 28, was necessary so that "those arriving from abroad ... don’t unknowingly spread the virus."
Britain has reported Europe's highest death toll linked to the coronavirus pandemic and its estimated infection rate remains higher than in most European nations.
"We have got the situation where - weeks ago - other countries put quarantine in and we didn't. Now, as everybody's lifting it, we're putting it in," opposition Labour leader Keir Starmer told London radio station LBC.
"A devastating effect"
Ryanair, easyJet and British Airways owner IAG wrote to the government threatening legal action to stop the "disproportionate and unfair" measure, warning that it would have "a devastating effect on UK’s tourism industry."
"We're seeing thousands of British families booking their holidays in Portugal, in Spain and Italy, but there's almost a collapse of inward bookings ... bringing those Europeans here to the UK, on which Britain's tourism industry depends," O'Leary told broadcaster ITV.
"What's irrational about it is all of those countries have a much lower Covid rate than the UK," he added.