Scientists have said going on a foreign holiday from the UK this summer is very much in doubt, as COVID rates spiral on the continent.
Professor Mike Tildesley, a member of a group that feeds into SAGE - which advises the government on coronavirus - said the possibility Britons will be able to head to the continent this summer was "extremely unlikely".
Another professor, University College London infectious disease expert Andrew Hayward, told Times Radio it was "unlikely" the government would want to encourage travel to countries with high rates of infection.
And Beate Kampmann, a professor of paediatric infection, immunity and international child health, told Sky News she believed international travel will remain "impaired" in light of the current rates around the world.
Until now, the government has left open the possibility that international travel could begin on 17 May, but said it will listen to the advice a group of experts who will report on the issue on 12 April.
Health Secretary Matt Hancock told Sky News that the government will stick to that plan, adding: "We'll make a judgement ahead of 17 May about whether it's safe to do so and set out more details when when we've seen the impacts a bit nearer the time.
"We have to be vigilant and we've set out the steps in the roadmap that there should be no international travel unless it's absolutely necessary until 17 May.
"The Global Travel Task Force will make a judgement and work with the industry on how quickly and whether we'll be able to be open and how we can reopen in a way that is safe. That's a judgement that we should make in a few weeks time, not now."
What's happening around the world?
Twenty countries in the European Union have now reported an increase in the rate of positive tests and 15 have said hospital or intensive care admissions have increased, according to the European Centre for Disease Prevention and Control.
In France, new restrictions have came into effect this weekend and will be in place for at least four weeks.
In Poland, where more people are on respirators than at any time since the beginning of the pandemic, most businesses and public venues are closing from the weekend for three weeks as a new lockdown begins.
Hungary has extended its lockdown for another week and in Bosnia, which has yet to start mass vaccination, a lockdown began in the capital on Friday.
It comes as Japan banned oversees visitors from the summer Olympics, which is due to take place in July and August this year.
Ministers have refused to set a date when tourists can travel abroad and, for now, foreign travel is banned unless there is an "essential" reason.
It has left thousands of people who want to travel abroad on holiday in late spring or summer in limbo over whether they can safely book flights and accommodation.
Prof Tildesley, a member of the Scientific Pandemic Influenza Modelling group, which reports into the government's Scientific Advisory Group for Emergencies (SAGE), said the risk of importing vaccine-resistant variants back into the UK would likely scupper the nation's annual getaway.
The professor of infectious disease modelling at the University of Warwick told BBC Radio on Saturday: "International travel this summer is, for the average holidaymaker, sadly I think, extremely unlikely."
He said the combination of overseas travel and the risk of importing new and more dangerous variants could undermine Britain's vaccine programme.
"We are running a real risk if we do start to have lots of people going overseas in July, for instance, and August, because of the potential for bringing more of these new variants back into the country," Prof Tildesley said.
Prof Hayward, meanwhile, told Times Radio: "I think the government has always been clear that travel abroad... any changes or plans are likely to change on that.
"It looks like some European countries are going to be having high levels and I think it's unlikely that we would want to encourage travel to those countries whilst they have high levels of infection."
And Prof Kampmann, director of The Vaccine Centre at the London School of Hygiene and Tropical Medicine, told Sky News: "Given what's happening, I'm sure there will be a delay [to international travel].
"And we've just heard the news from Japan... that's the only solution really that can be proposed. I think international travel will remain impaired.
"The more people we are vaccinating, the better."
Europe's failure to match the UK's vaccination programme has been cited as one of the reasons why infection rates in many EU countries are on the rise again.
The government's roadmap out of lockdown says The Global Travel Taskforce will report on 12 April with recommendations aimed at facilitating a return to international travel as soon as possible while still managing the risk from imported cases and variants of concern.
Following that, the government has said it will determine when international travel should resume, which will be no earlier than 17 May, but was thought likely to be closer to 21 June - the last date in the roadmap.
The latest developments across the Channel and beyond have raised the possibility travel may not restart on 21 June. It also reduces the likelihood that the UK will bring to an end sooner rather than later its 'red list' of countries from where people have to quarantine as they arrive from abroad.
Countries like Greece, as well as the hard-hit travel and aviation industries, had hoped EU vaccination certificates could have allowed Britons to go abroad on holiday this summer.
Last summer, reports say large numbers of COVID cases were imported into the UK by holidaymakers from Greece, in particular, but also Spain and Croatia.
A ongoing ban on foreign travel is likely to see further pressure on the prices of staycations, as more people are forced to take their holidays in the UK, with accommodation already reported to be up by an average of 35% compared to last year.