The new Brazilian coronavirus variant is already thought to be in the UK.
Head of Department of Infectious Disease at Imperial College London, Professor Wendy Barclay believes the mutant strain has already been detected here.
Travellers were banned from entering the UK from South America on Friday in a bid to avoid a new strain of the virus.
The travel ban also covers Panama and Portugal.
Although different from the new strains detected in the UK and South Africa, the Brazil variant shares some key mutations.
Transport Secretary Grant Shapps told the BBC this morning the travel ban was a "precautionary measure" and he was "not aware" of any cases being detected in the UK.
But Prof Barclay said one of the Brazilian variants had already been detected.
She said: "There are two different types of Brazilian variants and one of them has been detected and one of them has not."
"In the databases, if you search the sequences, you will see that there is some some evidence for variants from around the world, and I believe including the Brazilian one, which probably was introduced some time ago," she added.
"And that will be being traced very carefully".
After being asked earlier today on BBC Radio 4's Today programme if the strain was currently in the country, Mr Shapp's replied: "Not as far as we are aware, I think, at this stage."
