UP to 100,000 Brits have five days to get home or get cleared by a Covid test.
The race is on to beat new border rules being imposed on Wednesday or Thursday.
After then, anyone arriving in Britain by plane, train or boat must present a negative PCR test taken up to 72 hours before travelling.
Transport Secretary Grant Shapps said the deadline gave those abroad a chance to get home if they could not be checked where they were.
The Department for Transport has yet to reveal the exact date for the introduction of the rules, which will involve all passengers heading for England and Scotland.
Mr Shapps said the requirement was “much more urgent” because of new strains of Covid.
But Heathrow boss John Holland-Kaye claimed that thousands would be stranded overseas owing to a lack of the necessary facilities in some countries.
The new regime is expected to spark a scramble for return flights among Brits in winter sun holiday destinations, such as the Caribbean, Dubai and Maldives.
Mr Holland-Kaye called on the Government to take the lead in creating a common international standard for testing to resolve confusing differences between nations.
