The Labour leader defended Dawn Butler after she branded the Prime Minister “the master of untruths and half-truths”.
Ms Buter was then told to leave the House of Commons on Thursday after refusing to withdraw the allegations.
The Brent Central MP had said: “Poor people in our country have paid with their lives because the Prime Minister has spent the last 18 months misleading this House and the country over and over again.”
She highlighted disputed claims made by the Prime Minister, including that the link between Covid-19 infection and serious disease and death had been severed.
“It’s dangerous to lie in a pandemic,” she said.
“I am disappointed the Prime Minister has not come to the House to correct the record and correct the fact that he has lied to the House and the country over and over again.”
It is not considered within the boundaries of parliamentary etiquette to call another member a liar.
Speaking on LBC, Sir Keir has now insisted she was right to do so.
He said: “I agree with what Dawn had to say. I think the Prime Minister is the master of untruths and half-truths, and Dawn was simply giving some examples of that.
“I think there’s a lot of people that feel that, you know, it’s the person who’s not telling the truth rather than the person who’s calling it out that ought to be on the hot-spot. So, I agree with Dawn on that.
“But I do want to, in fairness to the temporary Speaker, Judith Cummins, who was there, she did the right thing, she followed the rules because Parliament doesn’t allow you to call other parliamentarians liars in the chamber, so I don’t criticise the Speaker … the deputy Speaker for what she did. She was following the rules. But do I support Dawn in what she said? I absolutely do.”
Ms Butler had been told to withdraw her comments, with Ms Cummins suggesting the MP may want to “correct the record”.
After she failed to do so, Ms Cummins then read out a statement in which she ordered Ms Butler to “withdraw immediately from the House for the remainder of the day’s sitting”.