Labour MP Dawn Butler was right to accuse Boris Johnson of lying in Parliament as he is the "master of untruths and half truths", Keir Starmer has said.
The former frontbencher was ejected from the Commons last week for refusing to withdraw her claim that the Prime Minister had "lied to the House and the country over and over again".
Accusing other MPs of deliberate deceit is forbidden under Parliament's strict rules.
But Ms Butler refused to take her comments back and was asked to leave for the rest of the day by temporary Deputy Speaker Judith Cummins.
The Labour leader told LBC: "I agree with what Dawn had to say. I think the Prime Minister is a master of untruths and half truths.
"Dawn was simply giving some examples of that and I think there's a lot of people who feel that it's the person who is not telling the truth rather than the person who is calling it out that should be on the hotspot. So I agree with Dawn on that."
But he said the Deputy Speaker was right to act as she was bound by Parliament's rules.
Ms Butler was ordered to withdraw from the Commons for the remainder of Thursday over her comments.
She 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."
Pointing to disputed claims made by the PM, including that the link between Covid-19 infection and serious disease and death had been severed, she added: "It's dangerous to lie in a pandemic.
"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."
Former Speaker John Bercow, who recently defected to Labour after criticising the PM, called for an overhaul to the rules to allow MPs to challenge each other for lying.
In a joint article with Ms Butler for the Times, Mr Bercow said: "The glaring weakness of the present system is that someone lying to tens of millions of citizens knows he or she is protected by an ancient rule. They face no sanction at all.
"By contrast, an MP with the guts to tell the truth is judged to be in disgrace. It is absurd and people can see that it is absurd."
The pair warned that the practice makes Parliament look weak, saying it was "bad and dangerous for democracy".
"Sooner or later, MPs of all parties who care for the truth, who care for our democracy, freedom and rights need to get off their bums and demand change," they said.
