Boris Johnson has said he will have "no hesitation" in moving countries around the red, amber and green lists of travel restrictions.
Asked whether there may be more countries added to the green list soon, where there are no requirements to quarantine on returning to the UK, the Prime Minister 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."
Despite the PM's remarks on travel restrictions, he said "at the moment" he can see nothing that shows step four of lockdown easing must be delayed - but warned the data is too "ambiguous".
Health Secretary Matt Hancock said there "isn't anything yet in data to say that we are definitively off track" to lift restrictions.
It comes despite Deputy Chief Medical Officer Jonathan Van-Tam suggesting medics would have a "ranging shot" at data on the Indian variant last week.
The PM admitted he's locked in "long" meetings just 11 days before the deadline to confirm or delay step four - which would lift all legal restrictions and reopen nightclubs.
Mr Johnson spoke as cases surge and the Delta Covid variant (formerly the Indian variant) is dominant in the UK.
Yet deaths, which lag by three to four weeks behind cases, fell to zero on Monday - prompting furious calls from Tory MPs to reopen the economy.
The Prime Minister said: “I can see nothing in the data at the moment that means we can’t go ahead with step four, all the reopening on June 21, but we’ve got to be so cautious.
“Because there’s no question the ONS data of infection rates is showing an increase.
"We always knew that was going to happen…. What we need to work is 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 just still ambiguous.
“And so every day we’re having long sessions, we’re interrogating all the data, we’re looking at all the various models, and the best the scientists can say at the moment in their guidance to us is we just need to give it a little bit longer.
“I’m sorry that’s frustrating for people, I know people want a clear answer about the way ahead for June 21, but at the moment we’ve just got to wait a little bit longer.”
