Seventy-seven countries are under review by government scientists for a possible move to the green list for quarantine-free foreign travel, it has emerged.
The Joint Biosecurity Centre (JBC) is understood to be reviewing the countries before ministers decide on a final list of new green destinations, due to be unveiled next week.
Green list status means unvaccinated travellers do not have to quarantine on their return, while amber list countries allow double-jabbed holidaymakers to visit them without being required to self-isolate when back home.
Government sources warned that many of the 77 might not make the cut once the assessment had been completed, but it provides the first insight into the size of the potential "long list" of countries whose Covid status means they can be rated as "possibles".
It comes as the Government is expected to confirm this week that double-vaccinated expats are set to be free to travel to the UK under plans to recognise foreign jabs from August 1.
Ministers are also working on proposals to open up quarantine-free travel to foreign nationals in a reciprocal deal with 33 countries which have recognised the NHS app. That could be announced as early as this week when the Department for Transport publishes the results of its review of the traffic light system for foreign travel.
Travel countries on the red, green and amber list
The prospect of a travel corridor between the US and UK, however, appeared to fade on Monday as the White House indicated that it would maintain its current ban because of concerns over the delta variant and a severe slowdown in its vaccination programme.
The travel industry – which is seeing bookings at 16 per cent of pre-pandemic levels – is concerned that the summer is slipping away amid fears that foreign travel this year could be lower than 2020.
Two experts consulted by The Telegraph suggested that between 20 and 35 countries merited inclusion in any new green list, which currently has 29 destinations including Malta, Madeira, Croatia, Bulgaria, Iceland, Gibraltar, Grenada, Barbados and Bermuda.
Robert Boyle, a former BA strategist, has "reverse engineered" the algorithm used by the JBC to rate countries, which suggests green destinations have case rate at or below 20, test positivity of 1.5 per cent or less and a testing rate of one or more per 100,000.
On that basis, he listed Austria, Germany, Italy, Latvia, Lithuania, Poland, Slovakia and Switzerland, Azerbaijan, Bhutan and Vietnam.
Paul Charles, the chief executive of travel consultancy The PC Agency, rated Austria, Bosnia, Canada, the Czech Republic, Germany, Hungary, Italy, Latvia, Lithuania, Poland, Romania, San Marino, Slovakia, Slovenia as likely green destinations.
He put Azerbaijan, Bhutan, Finland, Egypt, Estonia, French Polynesia, Jamaica, Japan, North Macedonia, Norway, Saudi Arabia, South Korea and Sweden on the green "watch list".
However, he suggested Malta, Madeira and Israel could all downgraded to amber from green or "green watch". No countries have yet come off the red list, requiring hotel quarantine for any arrivals, but Mr Charles said Kenya, Bahrain, India and Pakistan should all be returned to amber.
"Many destinations have been deserving of greater green status for some weeks so, while welcome, these upgrades are also long overdue," he added.
There are also growing hopes that France will be freed from its amber-plus status, which requires even double-jabbed travellers to self-isolate at home for 10 days, and be reinstated as an amber destination. Ministers are understood to be reluctant to add Spain to either amber-plus or red amid concerns over rising beta variant cases.