Boris Johnson has hinted some holiday destinations could be removed from the UK’s green list.
There are currently 12 countries, including Portugal and Israel, which people can travel to without having to quarantine when they return home.
The government is expected to issue an update to its international travel rules on Thursday.
Asked if more countries would be added to the green list, Johnson said he had to be “cautious”.
He pointedly only mentioned countries being moved up a Covid risk level, not down.
“We’ve got to continue to put countries on the red list, on the amber list, when that is necessary,” the prime minister said.
“We will have no hesitation in moving countries from the green list, to the amber list, to the red list, if we have to do so.”
He aded: “The priority is to continue the vaccine rollout, to protect the people of this country.”
Travellers returning to Britain from an amber location must quarantine at home for 10 days and take a pre-departure test and two post-arrival tests.
People arriving from a red list country have to quarantine in a government-approved hotel.
The red list currently consists of 43 nations from which arrivals are deemed to present a high risk of importing Covid.
Passengers arriving at Heathrow on direct flights from red list nations will be taken to a dedicated terminal.
Johnson’s caution about international travel came as he said people in England would have to “wait a little bit longer” before finding out if lockdown will end on June 21 as planned.