Mr Khan, who said he had yet to decide where to take his own summer holidays, told the Standard that he was “concerned” at the number of holidaymakers heading to Spain since the “traffic lights” system was introduced on May 17.
More than 55,000 people were estimated to have flown from England to Spain in five days last week, according to flight data company Cirium.
There were 1,344 flights to amber list countries between last Wednesday and Sunday, of which 284 went to Spain – treble the number at the start of May.
People returning from amber list countries are required to quarantine at home for up to 10 days and take two covid tests. Spain lags well behind the UK in its vaccination programme, with only 35 per cent of adults having a first jab.
Mr Khan said: “If it’s possible, if you want to go on holiday internationally, go to a green list country. Don’t risk an amber country or a red country.”
Green list countries include Portugal, Iceland and Israel. There is no need to quarantine but a covid test must be taken two days after returning to the UK.
Spain today lifted its own restrictions on UK travellers, with arrivals no longer needing a negative test. It hopes to move onto the UK’s “green” list on June 7.
Mr Khan said: “What I would say to Londoners is: respect the science. The science tells us that even though we may have got a grip with the virus in our country, if you go overseas to a country that hasn’t got a grip, you may inadvertently come back with a different strain of the virus.
“And if it’s in big numbers, we should be really worried. We know that with this virus, there are new iterations of the variants – South African, Kent, now Indian – and there will be other things going forward. We know this from Sars and from previous viruses.
“That is why I say it is really important we understand that and we don’t take risks we need to take.
“I think the Government made a mistake with the lack of clarity on the amber countries. I’m concerned when I see the numbers of people going to Spain, which is an amber country.”
He urged Londoners to get vaccinated to protect against the strains – but said he feared a new variant could prove more resistant than those already in circulation in the UK.
More than 4.1 million Londoners have had a first dose, of which 2.3m have had both doses.
Mr Khan added: “Will it work with other variants going forward? That is why it is really important we don’t take risks we don’t need to take. We are a global city. We need international travel. But we want it done safely.”
It came as a City Hall rfeport warned that the number of jobs in London will take two years to return to pre-pandemic levels.
It said there were 201,000 fewer workers on company payrolls than a year ago, down 4.8 per cent – about double the rate of decline seen elsewhere in the country.
But it predicted that “strong” economic growth of 5.4 per cent was likely in the capital this year, followed by a 6.9 per cent jump next yeat, though the absence of international tourists will limit the degree of bounce-back and limit the recovery of the hospitality, culture, leisure and retail sectors.
Today’s GLA London economic outlook report prompted Mr Khan to urge the Government to offer “essential” ongoing support for firms and workers once the furlough scheme ends in September.
It said employment levels may not recover until the second quarter of 2023 – adding to concerns after the capital’s unemployment rate was the worst of any UK nation or region.
Last week the London Chamber of Commerce called for retraining schemes for “mature” London workers, amid fears the capital’s unemployment rate could hit 9.4 per cent by December, leaving 464,000 people out of work.
Consumer spending by overseas tourists in central London was £7.4bn lower last year than normal and this “negative shock” is expected to persist until international travel recovers.
Mr Khan hopes his £6m Let’s Do London campaign, launched on the first day back at work after his re-election as mayor, will encourage “staycations” that will act as a stop-gap boost.
He said: “I’m optimistic that much of London’s economy will see strong levels of growth this year, and we will bounce back from this economic crisis like we have with previous economic shocks.
“But what these new figures show today is that with international tourism and footfall not likely to return to pre-pandemic levels this year, jobs in several key sectors still remain at risk when the furlough scheme comes to an end.”