Labour leader Sir Keir Starmer is self-isolating for the third time, after coming into contact with someone who tested positive for coronavirus.
He said he would be working from home until next Monday after being notified of the contact earlier.
Sir Keir confirmed on Twitter that he had no symptoms.
The Labour leader last self-isolated in December after a member of his staff tested positive for Covid-19, but he never showed any symptoms of the virus.
He also self-isolated in September after a member of his family showed symptoms - but they later tested negative, allowing Sir Keir to get back to Westminster.
If you are contacted by NHS Test and Trace and told you have been in contact with someone who has tested positive for the virus, you have a legal obligation to self-isolate.
You then have to stay at home, not going out for any reason, for 10 days from the time you last saw the contact.
If you don't stick to the rules, the police can issue you with a fine, starting at £1,000.
This morning I was notified that I must self isolate after coming into contact with someone who has tested positive for COVID-19.— Keir Starmer (@Keir_Starmer) January 25, 2021
I have no symptoms and will be working from home until next Monday.
For Sir Keir, he needs to stay indoors until next Monday and cancel all his upcoming plans for the week.
He will still be able to take part in Prime Minister's Questions on Wednesday via video link.
The current list of MPs set to question Boris Johnson, shows that only one will now physically be in the Commons with the PM.
'Most important part'
A number of politicians have had to self-isolate during the pandemic, including the prime minister.
The latest was Health Secretary Matt Hancock, who got a notification from the NHS app to stay at home.
He had the virus last March, but said self-isolation was "perhaps the most important part of all the social distancing" and urged others to do the same if contacted.
Mr Hancock's isolation period was due to end on Sunday, so he is expected back in Whitehall this week.