His deputy, Angela Rayner, will take his place at the despatch box.
This is the sixth time the Labour leader has had to isolate since the start of the pandemic – and the second time Sir Keir has tested positive for the virus.
He last tested positive for the virus in late October, and had to pull out of PMQS at the last minute. He was also unable to respond to Rishi Sunak’s Budget.
This story is being updated…