Boris Johnson has warned that the government will have "no hesitation" in moving green countries to the amber or red lists if cases rise.
The Prime Minister was asked if more countries would be added to the green list soon, meaning that people wouldn't have to quarantine on their return to the UK.
Mr Johnson said: "You've got to wait and see what the Joint Biosecurity Centre say and what the recommendations are about travel.
"We're going to try ... to allow people to travel, as I know that many people want to, but we've got to be cautious and we've got to continue to put countries on the red list, on the amber list, when that is necessary.
"I want you to know 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.
"The priority is to continue the vaccine rollout, to protect the people of this country."
Mr Johnson gave fresh hope that the June 21 unlocking would go ahead as planned, but warned that people still needed to be cautious.
Today he said there is still "nothing in the data at the moment that means we cannot go ahead with Step 4" of lifting coronavirus restrictions.
But infection rates are rising and Mr Johnson continued: "But we've got to be so cautious.
"We always knew that was going to happen, what we need to work out is to what extent the vaccination programme has protected enough of us, particularly the elderly and vulnerable, against a new surge, and there I'm afraid the data is still ambiguous.
"The best the scientists can say at the moment is we just need to give it a little bit longer."
