A two-day EU leaders’ summit due to start on Thursday has been postponed until 1 October after a security guard tested positive for Covid-19.
European Council President Charles Michel has to quarantine, as he and the guard were “in close contact early last week”, spokesman Barend Leyts tweeted.
Mr Michel has tested negative for the virus, but is self-isolating in accordance with the rules in Belgium.
Leaders were due to discuss foreign affairs, such as the crisis in Belarus.
Belarus has been gripped by weeks of mass protests, triggered by an election widely believed to have been rigged in favour of long-time leader Alexander Lukashenko.
The summit was also expected to be dominated by another crisis in the eastern Mediterranean, where Turkey and Greece have been in a standoff over gas drilling.
In a statement, Mr Leyts said: “[Charles Michel] today learned that a security officer, with whom he was in close contact early last week, tested positive for Covid.
“The president is tested regularly and tested negative yesterday. Respecting Belgian rules, he has gone into quarantine as of today.”
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Cases of coronavirus are surging across Europe.
On Friday, Belgium reported more than 2,000 new cases in a 24-hour period – a sharp spike, and its highest single-day increase since the first peak of cases on 15 April.
In Belgium, anyone who comes into contact with someone who is a confirmed case of coronavirus has to isolate at home for 14 days.