Prime Minister Boris Johnson on Wednesday said the government was looking at ways to stop a variant of the coronavirus found in Brazil from entering Britain.
Japan's Health Ministry on Sunday said it detected a new strain of Covid-19 in four travellers from Brazil's Amazonas state.
The strain featured 12 mutations, including one also found in highly infections variants discovered in Britain and South Africa.
"We are concerned about the new Brazilian variant ... and we're taking steps in respect of the Brazilian variant," Mr Johnson told a parliamentary committee.
"I think it's fair to say that there are lots of questions we still have about that variant."
Sir Patrick Vallance, the UK's chief scientific adviser, said the Brazilian mutation appeared to have some features of the other coronavirus variants.
"What we're seeing is that mutations are cropping up across the world, which are quite similar in terms of the changes," Mr Vallance told ITV.
He said the Brazilian strain appeared to have similarities to the South African one.
Mr Vallance said there was no evidence that any of the new variants made the disease more severe.
"The changes that we're seeing with the variants are largely around increased transmission," he said.
"It makes it easier to get it from one person to another. It makes it easier therefore to catch."
Mr Vallance said there was no evidence that vaccines would be ineffective against the strain, which has fuelled a surge in infections in Britain.
But they did not know for sure if that would be the case with the South African or Brazilian strain.
"There's a bit more of a risk that this might make a change to the way the immune system recognises it but we don't know," Mr Vallance said.