UK Prime Minister Boris Johnson was accused of “kicking the can down the road” after he refused to give details on the return of international travel.
Mr Johnson said the country would move on to the next stage of its roadmap out of lockdown, with some hospitality venues reopening outdoor areas next week.
However, he gave little information on the resumption of non-essential international travel from Britain, despite demand for summer holidays abroad and many expats keen to visit the UK without having to quarantine in a hotel.
The prime minister would not commit to a May 17 deadline to restart overseas trips, and said Britain should not "underestimate the difficulties that we're seeing in some of the destination countries".
Boris Johnson visits UK AstraZeneca plant - in pictures
"Obviously we are hopeful that we can get going from May 17, but I do not wish to give hostages to fortune,” he said in a TV address to the nation.
"I wish I could give you more on that. I know that people watching will want to know exactly what they can do from May 17, but we're not there yet. As soon as we have more solid information, more solid data, we'll let you know."
A UK government review of travel said that it was not yet clear if tourists would have to wait beyond May 17 for the outbound restrictions to be lifted.
Ministers last week proposed a traffic light system, which will rank different countries according to their inoculation rates and numbers of Covid-19 infections.
Virgin Atlantic chief executive Shai Weiss called for the US should be added to the "green list", while British Airways boss Sean Doyle predicted quarantine-free travel would be allowed for many countries by summer.
But the government urged people not to book summer holidays, saying it was "too early to predict" to which countries travel would be permitted.
The lack of clarity from Mr Johnson disappointed Britain's biggest airport and travel companies, which said the tourism sector was teetering on the brink of collapse.
Heathrow Airport chief executive John Holland-Kaye said a clearer timeline for the return of international travel was needed.
"It's disappointing that the opportunity has been missed to provide more certainty to reunite families separated by travel restrictions, to give sunseekers the confidence to book ahead for their summer getaway," he said.
Paul Charles, co-founder of the Save Our Summer campaign, said the industry needed certainty.
“Government cannot keep kicking the can down the road as hundreds of thousands of jobs are at risk,” he tweeted.
“It needs to urgently put in place safe solutions to travel for business, leisure and to see family, and not backtrack on dates for updates.”
Noel Josephides, chairman of travel group Sunvil, said the sector would need weeks of notice to reopen hotels and resume flight routes.
"It is not possible to launch a tour operation with one or two weeks' notice," he said.
Under the traffic light system, countries that could make it on to the green list, with no mandatory quarantine, include those with high inoculation rates such as Israel and the US.
Destinations on the amber list would require self-isolation, and hotel quarantine would feature for those listed red.
Mr Johnson said that the government was looking at a Covid-19 status certification system, or vaccine passport, to help reopen larger events, as well as for travel.
He said there "absolutely no question" of people being asked to provide Covid certification to go to shops or restaurants, but vaccination passes would possibly be used for travel.
Virus passports are an idea backed by many tourist-dependent countries and airlines but opposed by more than 70 British members of Parliament.
Vaccines Minister Nadhim Zahawi said the government would ensure there was a system governing Covid passports when international travel resumes.
“For international travel, if our citizens need to travel at the right time when international travel returns we will make sure they are able to demonstrate their status – through a pre-departure test or a vaccination certificate,” he told Sky News on Monday.
“Domestically, there will be absolutely no issue around pubs or restaurants requiring any form of certification.”
The main opposition Labour party said Covid passports were discriminatory.
“I cannot support a policy in which you need a vaccine ID card in order to get into H&M or Next,” shadow health secretary Jonathan Ashworth told the BBC.
The UK has administered more than 31 million vaccinations and 5 million second doses, at a pace that has far outstripped that of popular holiday destinations such as France.
But scientists said the effect of vaccinations on virus transmission was still unclear as they warned a third wave of coronavirus was possible if most restrictions are lifted on June 21 as planned.
Prof Graham Medley from the London School of Hygiene and Tropical Medicine, scientific adviser to the government, said it was “almost inevitable” there would be a rise in cases later in the year.
“We do know because the vaccine isn’t 100 per cent effective and there will be some transmission and some breakthrough of immunity,” he told the BBC.
“The amount of infection and death is dependent not only on the vaccine but on what people actually do. It’s quite likely we will have to see some kind of measures to reduce transmission for a long time.”
On Monday, the UK reported 26 new deaths within 28 days of a positive test and 2,762 new cases.
Free test kits will be made available from pharmacies, community centres and home delivery services on Friday.
The government believes rapid testing of the whole population and a system of Covid-19 status certification will help to maintain control of the virus when restrictions are eased.
England will test the vaccine passport at a comedy club, a nightclub and Wembley stadium as authorities try to determine how to host mass events safely.
The government said a Covid-status certification system was being developed.
This would show whether a person has had a vaccine, a recent negative test or natural immunity from a positive test taken in the past six months.
The trial, to be run at nine events, will be used to assess whether large events can be held in closed settings without social distancing.
Cabinet Office Minister Michael Gove said certification for international travel was “an inevitability” and it could also be a “valuable aid” in reopening parts of the domestic economy faster.
"Unless the government takes a lead, we risk others establishing the rules of the road," Mr Gove wrote in the Sunday Telegraph.
Before the announcement, 39 countries were on the red list, including the UAE, southern African nations and all of South America.
Last week, scientists pushed for the ban on international travel to remain in force.
Anne Johnson, professor of epidemiology at University College London, said that importing new coronavirus variants was “one of the biggest risks” facing the UK.
“This is a risk where you’ve got high rates of infection. I’m for staycations,” she said.