The UK must be “realistic” over the resumption of foreign holidays due to “a surge” of coronavirus in popular destinations, Boris Johnson said.
The Prime Minister told a Downing Street briefing “I wish I could give you more on that” but insisted there was not enough “solid data” on the virus.
This led to an angry reaction from the travel industry.
Heathrow Airport chief executive John Holland-Kaye claimed “the opportunity has been missed to provide more certainty”, while trade body the Business Travel Association described the update as “beyond disappointing”.
A Downing Street paper on easing coronavirus restrictions confirmed that a traffic light system for international travel will be introduced.
But it stated it is not known when trips could resume, and advised consumers not to make bookings yet.
People living in the UK are banned from taking foreign holidays.
The Government’s Global Travel Taskforce will provide a report later this week on how international leisure travel can resume.
Under England’s road map for easing pandemic rules, the earliest that foreign holidays could be permitted is May 17.
Mr Johnson said: “Obviously we are hopeful that we can get going from May 17, we are hopeful.
“But I do not wish to give hostages to fortune or to underestimate the difficulty we are seeing in some of the destination countries people might want to go to.
“We don’t want to see the virus being reimported into this country from abroad.
“Plainly, there is a surge in other parts of the world and we have to be to be mindful of that and we have to be realistic.”
Mr Johnson said the Government will set out “well before May 17th what we think is reasonable”.
He added: “I know that people watching will want to know exactly what they can do from May 17th but we’re not there yet.
“As soon as we have solid information, more solid data, we’ll let you know.
“But that’s where we are for the time being.”
Under the traffic light system, assessments will be based on a range of factors, including the proportion of a country’s population which has been vaccinated, rates of infection, emerging new variants and the country’s access to reliable scientific data and genomic sequencing.
Travellers arriving from countries rated “green” will not be required to isolate although pre-departure and post-arrival tests will still be needed.
For people who have been in countries classed as “amber” or “red”, arrivals will be required to isolate or enter quarantine.
The document stated it is “too early to say” which countries will be on the green list.
It added: “For the moment, the Government advises people not to book summer holidays abroad until the picture is clearer.”