Scientists have voiced anxiety following the announcement that people arriving in the UK from the EU and the US will not have to quarantine if they have been double-jabbed.
Under current rules people arriving from amber list countries are required to isolate for 10 days after setting foot in the UK.
Health Secretary Sajid Javid said it would be "another step on the road to normality" - but critics warn it could open the door for mutant strains of the virus to make their way into the country.
The quarantine requirement will be lifted at 4am on August 2, the government announced yesterday.
But it has sparked warnings that vaccinated people can still spread Covid.
Prof Christine Pagel, director of the Clinical Operational Research Unit at UCL, told the Guardian : “By definition the variants we are most worried about are the ones that infect vaccinated people.
“So making vaccinated people exempt from quarantine doesn’t make any sense if variants are what we are most worried about.
“If the current reduction in cases here is real and if we’re actually in a good place, then why would we risk it?”
She added that the government should be "taking advantage of the fact that we are an island" with control over variants coming in.
Prof Pagel said that a new variant would spread "everywhere".
Also sounding a note of caution was Mike Tildesley, professor of infectious disease modelling at the University of Warwick.
He told Times Radio there is "a risk attached" to easing self-isolation requirements.
But he added: "If people are double vaccinated, if people are coming from countries that have similar levels of infection to ours, and no real risk of variants of concern that might be introduced into the country, then it's probably a practical decision that has to be made to try to support the tourist industry."
Following an encouraging few days in which Covid-19 case numbers in the UK have fallen from above 50,000 on July 17 to 27,734 by 9am on Wednesday, Labour sounded warnings over the impact of the travel changes.
Shadow transport secretary Jim McMahon said: "The Government's track record on our borders has been one of recklessness and confusion.
"They are in danger of continuing this by setting out changes in policy, applying to England only, without the scientific data and criteria we need to make sure we don't see another Johnson variant run rampant through the country and damage the effort of the British public.
"We want to see international travel opened up safely.
"Ministers need to be clear on what progress has been made on reaching reciprocal agreements for Brits travelling abroad - particularly regarding the NHS app being accepted as proof of Covid status.
"We also need a clear green and red list and the country-by-country data to back it up."
Travel companies have warned that unless the UK is opened up to visitors from abroad, catastrophic job losses will follow.
Paul Charles, chief executive of travel consultancy The PC Agency, claimed allowing fully vaccinated US and EU citizens to enter the UK without going into quarantine "would finally be the dawn of a consistent global policy".
He said he is one of several people in the sector who have made it "very clear" to Cabinet and junior ministers that there will be "thousands of job losses" in the UK across aviation, hotels and visitor attractions if travel is not "opened widely" in August.
He added: "Foreign visitors are essential to UK plc."
Earlier today The Mirror reported that huge discrepancies have opened up in rates of vaccination around the world.
This has sparked concern about variants spreading around the world, as richer nations are criticised for failing to share jabs.
Health and Social Care Secretary Sajid Javid said: "Our vaccination programme is building a wall of defence against this virus so we can safely enjoy our freedoms again, with 7 in 10 adults in the UK now double jabbed.
"By reopening quarantine-free travel for travellers who have been fully vaccinated in European countries and the USA, we’re taking another step on the road to normality which will reunite friends and families and give UK businesses a boost."
Earlier today Foreign Secretary Dominic Raab admitted the Government "cannot guarantee" that US and EU travellers will not try to show fake vaccination certificates.
He told BBC Radio Four's Today programme: "We can't guarantee that some people might not do it. I think it is highly unlikely."
Mr Raab said there is a "double lock" of written certification and proof of US residency for American travellers, which he said could allow "further checks if there is any suspicion of fraud".
Describing the new approach as "smart, sensible", he added: "Both domestically with our rollout but also internationally, we want to open up, we want to move the country forward, but we want to do it irreversibly and we need to take solid, surefooted steps forward.
"We feel this is a modest opening up of international travel, but one which has the reassurances that means that we can take further steps forward as we build confidence in the system."
