Prime Minister Boris Johnson has said he is "very concerned" about a new variant of coronavirus which has been identified in Brazil.
Mr Johnson, speaking at the Liaison Committee, insisted to senior MPs that the government was "taking steps" to ensure the new strain is not imported to the UK, but he did not specify what measures were being taken.
He added how experts are looking into whether new strains of the virus - including one first identified in the UK and another first spotted in South Africa - are resistant to vaccines.
He said there is no evidence so far to suggest this is the case.
Home Affairs Committee chairwoman Yvette Cooper grilled the PM over border measures saying the nation could have "stronger quarantine and stronger checks like very many other countries do".
"We were warned about the Brazil variant three days ago, we don't know yet whether that will potentially undermine the vaccination programme so why aren't you taking immediate action on a precautionary basis?" she asked.
The PM responded: "Well we are and we're putting in extra measures to ensure that people coming from Brazil are checked and indeed stopping people coming from Brazil."
Watch the PM's appearance at the Liaison Committee in full:
Ms Cooper asked: "Are you, that's a new announcement?"
"No sorry I said what we were looking at was the question of whether or not the Brazilian variant is immune resistant," he said.
Pressed on whether he was suggesting a new travel ban would be imposed on Brazil, Mr Johnson said: "We are taking steps to ensure that we do not see the import of this new variant from Brazil."
Ms Cooper repeatedly pressed on what those steps exactly are, to which Mr Johnson cited the incoming plans for negative tests before departure being needed.
The requirement for a negative test before entering the UK will take effect from Friday.
The PM added that strain on the NHS is "colossal", with a "very substantial" risk that England's intensive care units will soon be overwhelmed.
His appearance comes after he refused to rule out bringing in tougher Covid-19 measures in England, amid concerns current lockdown restrictions are not enough to stem the spread of a new, fast spreading variant of the virus.
Pressure to tighten restrictions was increased on Wednesday afternoon when Scottish First Minister Nicola Sturgeon announced stricter laws would be enforced north of the border.
Mr Johnson earlier faced questions from Labour leader Sir Keir Starmer at PMQs, in which he was accused of responding too slowly to developments in the pandemic.
The Liaison Committee, which is made up of chairs from the other Commons Select Committees, is the only one that gets to quiz the prime minister.
Wednesday's appearance is only the third time Mr Johnson has answered questions from the committee, despite prime ministers being expected to attend at least three times a year.
The following Chairs were expected to participate in the hearing:
Clive Betts, Housing, Communities and Local Government Committee
Robert Halfon, Education Committee
Jeremy Hunt, Health and Social Care Committee
Darren Jones, Business, Energy and Industrial Strategy Committee
Catherine McKinnell, Petitions Committee
Caroline Nokes, Women and Equalities Committee
Stephen Timms, Work and Pensions Committee
Hilary Benn, Committee on the Future Relationship with the European Union
Sir William Cash, European Scrutiny
Yvette Cooper, Home Affairs Committee
Philip Dunne, Environmental Audit Committee
Simon Hoare, Northern Ireland Affairs Committee
Tom Tugendhat, Foreign Affairs Committee
Pete Wishart, Scottish Affairs Committee