This marks the fifth time Sir Keir has been forced to isolate, having already been in quarantine due to a close contact with a positive case four times previously.
He was forced to isolate in September and December last year, and again in January and July, when one of his children tested positive in the summer.
However, this is the first time the leader himself has tested positive for the virus.
It is understood that Sir Keir had been taking daily lateral flow tests and received a positive result just before Prime Minister’s Questions (PMQs).
Who will take his place at the Budget announcement?
The leader of the opposition normally responds to the Chancellor’s statement in the Commons after the Budget announcement.
As Sir Keir is unable to attend, his place at the dispatch box will now be taken by Shadow Chancellor Rachel Reeves.
Labour’s deputy leader Angela Rayner is unable to take his place as she remains on bereavement leave.
Shadow Business Secretary Ed Miliband will be standing in for the Labour leader at PMQs.
Ahead of the Budget announcement today (27 October), Sir Keir tweeted: “The Budget must take the pressure off working people.
“With costs growing and inflation rising, Labour would cut VAT on domestic energy bills immediately for 6 months.
“Unlike the Tories, we wouldn’t hike taxes on working people and we’d ensure online giants pay their fair share.”
When is the Budget announcement?
Chancellor Rishi Sunak will deliver the Autumn Budget from around 12.30pm in the House of Commons.
The Budget announcement typically lasts for an hour, with shadow chancellor Rachel Reeves putting forward the opposition’s response to the government’s proposals immediately afterwards.
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