Michel Barnier, the EU’s Brexit negotiator, has tested positive for coronavirus.
He wrote: “I would like to inform you that I have tested positive for #COVID19.
“I am doing well and in good spirits. I am following all the necessary instructions, as is my team.
“For all those affected already, and for all those currently in isolation, we will get through this together.”
The news prompted a number of messages of goodwill, including from European Commission President Ursula von der Leyen and EU Council President Charles Michel.
Ms von der Leyen will be tested for COVID-19 following Mr Barnier’s diagnosis, commission spokesman Eric Mamer said.
The pair last met two weeks ago and so far Ms von der Leyen has not displayed any symptoms.
The worldwide COVID-19 pandemic has resulted in negotiations between Britain and the EU about a future trade deal scheduled for this week being cancelled.
Despite this, the UK and EU did exchange draft legal texts for the future relationship between the two sides on Wednesday.
Prime Minister Boris Johnson has insisted that he will not ask for an extension to the Brexit transition period, due to come to an end in December, because of coronavirus.
“It’s not a subject that’s being regularly discussed, I can tell you, in Downing Street at the moment,” he said.
“There is legislation in place that I have no intention of changing.”
Negotiations are due to resume next week, but they might not take place in person.
The PM’s official spokesman said: “We of course send Michel Barnier our best wishes.
“In relation to Brexit negotiations… we have been in close conversation with the EU about looking at ways to continue progressing the negotiations.
“Both sides have shared their texts of potential legal agreements.”
Foreign Secretary Dominic Raab said the government was still committed to striking a post-Brexit free trade deal with Brussels by the end of 2020.
“The negotiating texts have been exchanged. I would have thought in the 21st century there is scope for this to be done through teleconferencing and remotely – at least for the period it is required,” he said.
“I won’t go any further than the PM at this point. We don’t want to extend, we want to get this done.
“We don’t want any more uncertainty than we have already had from this process.”