Labour leader Sir Keir Starmer is self-isolating after a member of his household "showed possible symptoms of the coronavirus", the party has said.
The person displaying symptoms has had a test and Sir Keir is now awaiting the results "in line with NHS guidelines", they added.
The Labour leader will not be speaking in Monday's Commons debate on the government's post-Brexit plans.
However, he is not reported to have shown any coronavirus symptoms.
Sir Keir - who is due to address the TUC Congress on Tuesday - found out about the concerns over the member of his household following an appearance on LBC radio on Monday morning.
A Downing Street spokesman said: "The prime minister has spoken to the leader of the opposition this morning and gave best wishes to him and his family."
The coronavirus infection rate is rising across the UK, with rules restricting the size of social gatherings coming into force in England, Scotland and Wales from Monday.
A Labour spokesperson said: "This morning Keir Starmer was advised to self-isolate after a member of his household showed possible symptoms of the coronavirus.
"The member of his household has now had a test. In line with NHS guidelines, Keir will self-isolate while awaiting the results of the test and further advice from medical professionals."
Sir Keir's family secured tests via his wife Victoria's workplace.
The hospital where she works, in occupational health, provides on-site testing for staff and their families who have symptoms.
The move comes ahead of what was to be one of Sir Keir's most important parliamentary performances since becoming Labour leader in April.
Instead shadow business secretary - and former leader - Ed Miliband will open for the party in the debate over the government's Internal Market Bill on Monday afternoon.
The legislation attempts to override parts of the Brexit withdrawal agreement reached by the UK and EU.
Last week, Commons Leader Jacob Rees-Mogg announced he was self-isolating at home after one of his children showed symptoms of coronavirus.
Prime Minister Boris Johnson was taken into intensive care in April after he caught it.