Michel Barnier, the EU's chief Brexit negotiator, has tested positive for coronavirus.
Barnier said he was "doing well and in good spirits" in a tweet announcing his diagnosis on Thursday.
His diagnosis throws the process of negotiating a post-Brexit trade deal between the EU and the UK into doubt.
Barnier had been due to hold talks in London this week, but they were cancelled due to the coronavirus pandemic.
The post-Brexit transition period, under which the UK will continue to follow EU rules, will end in December and Prime Minister Boris Johnson has vowed not to extend it.
