Returning holidaymakers should be tested twice for coronavirus to reduce the quarantine period, Keir Starmer has said.
The Labour leader urged ministers to consider testing people at airports when they return to the UK and then testing them again a few days later.
British tourists in Spain faced chaos when the Government abruptly decided to scrap quarantine-free travel with the country at the weekend - forcing people to isolate for 14 days on returning home.
Cabinet Minister Oliver Dowden said there was "no viable alternative" to the two-week quarantine period for returning tourists, and insisted testing people at airports was not a "silver bullet".
However Mr Starmer pointed to evidence suggesting the 14-day isolation period could be reduced with multiple tests.
Speaking on a visit to Falmouth, the Labour leader said: "There's some evidence it could be shortened to 10 or nine days but that depends on really effective testing and that is why we have pushed the Government so hard on testing.
"There's the capacity to test, the Government needs to use that to test on arrival and then after a short interval because if that period of 14 days can be brought down to eight, nine or 10 then obviously there's a huge benefit in that."
Mr Starmer said Labour supported the need for quarantine measures but he added: "It's really unfortunate for those in other countries and I really feel for them but it is necessary that we take all preventative measures to prevent a second spike."
Heathrow boss John Holland-Kaye pleaded with the Government to allow tests to ease restrictions - and said the airport would be "ready to go in two weeks".
He said: "We've started to mobilise on that so we can be ready as fast as possible.
"The speed really depends on the Government and them putting the legislation in place to allow people to come out of quarantine early if they have passed two tests, and deciding whether the second test has to be after five days or eight days or some other time."
Heathrow is working on a system where passengers would be charged £140 a head for a test.
Ministers are worried that testing people at airports will not work, as coronavirus can incubate for a period of time before it is detectable.
Mr Dowden told the Today programme: "We are not at the point where there is a viable alternative to the 14-day quarantine.
"There is a real risk here - the virus is spreading around the world, it's rising rapidly around the world.
"We need to ensure that the measures we've taken in the UK - which have been very difficult - to keep this virus under control, do not go to waste because we allow cases to come in from elsewhere."
But early modelling from the London School of Hygiene and Tropical Medicine suggested 94% of coronavirus cases would be detected if the quarantine period was cut to eight days and passengers tested negative on the seventh.
