One of two Brazilian coronavirus variants has already been detected in the UK, a leading virologist said.
The variant is distinct from strains that emerged in south-east England and South Africa but they share some key mutations.
One strain is understood to be more infectious than the other because it is better able to attach itself to human cells.
Prof Wendy Barclay, who is advising the government as head of the G2P-UK National Virology Consortium, said the less infectious variant probably arrived in the UK "some time ago".
"In the databases, if you search the sequences you will see that there is some evidence for variants from around the world and, I believe, including the Brazilian one, which probably was introduced some time ago," she said.
"And that will be being traced very carefully."
She later clarified: "The new Brazilian variant of concern, that was picked up in travellers going to Japan, has NOT been detected in the UK. Other variants that may have originated from Brazil have been previously found."
Travellers from South America and Portugal, which has strong links with Brazil, were banned from entering the UK from Friday.
Transport Secretary Grant Shapps said he was not aware of any cases of the Brazilian coronavirus variant in the UK.
Mr Shapps said the travel ban was a precautionary measure to ensure Britain's vaccination campaign is unhindered by new variants.
"Our scientists aren't saying that the vaccine won't work against it ... (but) we do not want to be tripping up at this last moment (of vaccine rollout) which is why I took the decision as an extra precaution to ban those flights," he told Sky News.
Dr Mike Tildesley, a leading epidemiologist, criticised the government for the delay in imposing the travel ban.
“We always have this issue with travel bans, of course, that we’re always a little bit behind the curve,” he told the BBC.
“My understanding is that there haven’t really been any flights coming from Brazil for about the past week, so hopefully the immediate travel ban should really minimise the risk.”
Japanese scientists first notified Brazilian authorities on Saturday about the highly infectious variant after detecting it in four travellers who arrived from the state of Amazonas.
Hospitals in Manaus, a major city located in Amazonas, are currently at breaking point amid reports of oxygen shortages and exhausted staff.