The Brazil coronavirus strain has been found in 15 countries that are not yet on the UK's red quarantine list.
The World Health Organisation revealed the P1 strain, first identified in the UK but originally from Brazil, has now been found in a total of 23 countries around the world.
Eight are on the list which means anyone arriving in the UK from there must quarantine for at least 10 days in a designated hotel - but the remaining nations are not.
This means travellers coming in from those 15 countries are able to self-isolate at home.
Health officials announced at the weekend they had found six cases of the Brazil variant - first seen in the city of Manaus - in the UK.
Two cases were found in the same household in South Gloucestershire, with one flying from Sao Paulo to London via Zurich.
Three were Scottish residents who flew from Brazil to Paris and then on to London and Aberdeen.
But the sixth person who tested positive for the strain had not filled in their test registration card.
Ministers said the development posed no wider threat to the public but there are fears the mutant strain may spread more rapidly and may respond less well to existing vaccines.
The countries where the strain has been found that are not on the red list are Belgium, Canada, Faroe Islands, Finland, France, Germany, India, Italy, Japan, Mexico, the Netherlands, Republic of Korea, Réunion, Sri Lanka and the US which also has the worst coronavirus death rate in the world.
The remaining countries - excluding the UK - are Argentina, Brazil, Colombia, French Guiana, Peru, Portugal, United Arab Emirates.
Nick Thomas-Symonds, Labour’s shadow home secretary, has called for the Government to tighten up border controls and bring in a "comprehensive hotel quarantine system" adding they have acted "too little, too late".
On Monday Health Secretary Matt Hancock said in a Downing Street briefing that the number of cases found in those countries were "exceptionally low" and the nations on the red list had higher numbers.
But the Government in Scotland has said it will quarantine everyone coming into the UK in hotels - no matter where they are from.
On Monday Boris Johnson defended the borders system saying it was "one of the toughest in the world" adding they had acted as quickly as possible.
It is thought to be more contagious and there is a chance it may not respond as well to the jab - although no vaccine data for it exists.
Danny Altmann, a professor of immunology at Imperial College, said indications show it is likely the Brazilian variant is "breaking through" antibodies built up by previous infection.
But Professor Stephen Powis, the NHS’s national medical director, has said coronavirus vaccines can be quickly adapted to tackle new strains.
Professor Powis told BBC News: “The new vaccines which are being used for Covid can be adapted very rapidly so it’s likely that if we do need to change the vaccine that can be done in months, rather than years, which was the case with the more traditional vaccines.”"
Officials are now desperately trying to identify and isolate the mystery person, and have appealed to anyone who took a Covid test on February 12 or 13 and has not received a result, or did not complete their test registration card, to call 119 in England or 0300 303 2713 in Scotland as soon as possible.