Foreign Secretary Dominic Raab is self-isolating after recently being in "close contact" with someone who tested positive for coronavirus.
The cabinet minister will have to self-isolate for a period of 14 days, in line with government and NHS rules.
Earlier on Thursday, the foreign secretary had called for new presidential elections to be held in Belarus following the publication of an independent report into August's contested vote.
Confirming Mr Raab's self-isolation on Thursday evening, a spokesman said: "The foreign secretary was today informed that an individual with whom he has been in recent close contact with has tested positive for coronavirus.
"In line with government regulations and NHS track and trace rules, the foreign secretary has taken immediate steps to self-isolate for the required period.
"He will continue to work remotely during this time."
It is understood Mr Raab has not shown any symptoms, while the individual who tested positive is not another minister, MP or member of the government.
Mr Raab, who also holds the title of first secretary of state, deputised for Boris Johnson earlier this year when the prime minister was in intensive care with COVID-19.
Around the same time of Mr Johnson's illness, Health Secretary Matt Hancock also tested positive for coronavirus, as did Sir Mark Sedwill, the then cabinet secretary and head of the civil service - although Sir Mark's illness was only revealed weeks later.
Downing Street this week stressed that Business Secretary Alok Sharma, like all ministers, was following COVID guidelines.
It recently emerged Mr Sharma travelled to South Korea on government business only days after coming into contact with staff who later tested positive for COVID-19.
But a spokesman for the business secretary said Mr Sharma "had no close contact with the individual prior to his departure and has not been told to self-isolate by NHS Test and Trace".
Mr Sharma attended Tuesday's cabinet meeting at the Foreign Office in person, where ministers were "socially distant" throughout, Number 10 said.
Earlier this week, Mr Raab said the UK government would "need to be patient and wait and see who wins the US election".
He added it was "important the process is given sufficient time to reach a conclusion" and the government had "full confidence in the checks and balances of the US system to produce a result".