Boris Johnson’s chief negotiator David Frost is self-isolating at home after showing "mild" symptoms of the coronavirus, Downing Street has said.

The top official has been leading the UK government’s efforts to forge a post-Brexit trade deal with the European Union before the transition period ends on 31 December 2020.

Mr Frost’s counterpart Michel Barnier, the EU’s chief negotiator, said on Thursday he had tested positive Covid-19. He said in a Twitter video message from his home in France that he is recovering well and is in good spirits.

No 10 said that Mr Frost was now self-isolating after experiencing symptoms of the virus – but it is not known whether he has been tested for the illness or whether the PM is among the recent contacts who might need to be traced for testing

A Downing Street spokesman said: "David has been showing mild symptoms so he is following the guidance to self-isolate. We remain in contact with the European Commission and expect further conversations between the teams next week."

Asked whether Mr Johnson had been with Mr Frost in recent days and might need testing, the spokesman said: "We are following the same protocol that has always been followed."

The two chief Brexit negotiators have not had contact with each other since the first round of trade talks ended in Brussels two weeks ago, on 5 March.

Talks planned between the two sides for this week have been postponed because of the outbreak, and UK government officials have talked about the possibility of holding discussions via videoconferencing.

At a time of huge uncertainty, Mr Johnson has been urged to extend the transition period – something he has thus far refused to consider.

The Lib Dems have accused him of “putting his head in the sand” over the threat posed by the virus, while Labour MP Ben Bradshaw told The Independent it was “reckless insanity” to carry on as if nothing had changed.

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Under arrangements between Britain and the EU, London would have until June to ask for an extension, and it would be possible for a maximum of two years.

“In a situation with major healthcare challenges in the short and long term and economic challenges already requiring urgent action, there will not be enough political time and attention to successfully conclude this EU-UK agreement,” said Fabian Zuleeg of the Brussels-based European Policy Centre think tank.