Boris Johnson has refused to admit to criminality in Downing Street as he was grilled by MPs after fines were issued over the police’s partygate investigation.

The prime minister came under pressure, during questions from the Commons liaison committee, to concede that this week’s announcement of 20 fixed penalty notices by the Met Police meant that the law had been broken.

But Mr Johnson stonewalled attempts by the SNP’s Pete Wishart to force him to do so or to admit to any breach of the ministerial code, telling him: “You are just going to have to hold your horses.”

Earlier, a spokesman for the prime minister had apparently refused to endorse comments by deputy PM Dominic Raab, who told Sky News that the fixed penalties were being issued to “those that have breached the regulations”.

Mr Wishart told the PM: “You can at least accept there has been criminality.

“You do accept it? Twenty fixed penalty notices have been issued for goodness sake. There can’t be any contradiction and doubt about criminality?”

Mr Johnson replied: “I have been, I hope, very frank with the House about where I think we have gone wrong and the things that I regret and I apologise for, but there is an ongoing investigation.

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“I understand the point you’re making but… I have been very clear I won’t give running commentary on an ongoing investigation.”

Labour has described it as “totally untenable” for “Downing Street to sort of refuse to acknowledge what is a statement of fact and law” and the Liberal Democrats said the position was “absurd”.

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Raab defends civil servants over partygate

Downing Street has said it would reveal if Mr Johnson – or Cabinet Secretary Simon Case – is issued with a fine but thus far has not made any such statement.

Mr Wishart put it to the PM that he would be “toast” if he was given a fixed penalty – but Mr Johnson again refused to comment.

The prime minister has also faced claims that he breached the ministerial code after telling parliament initially that no rules had been broken in relation to the parties.

During Prime Minister’s Questions earlier, Labour leader Sir Keir Starmer said of Mr Johnson: “He told the House no rules were broken in Downing Street during lockdown.

“The police have now concluded there was widespread criminality.”

He said that according to the ministerial code, ministers who knowingly mislead the House of Commons should resign and asked, “why is he still here?”

But Mr Johnson told the select committee: “I think it’s very important that you should be clear with the House of Commons – and I’ve tried my best to be as clear as I can about my understanding of events.”

Earlier on Wednesday, Mr Raab told Sky News “there were clearly things that were got wrong” and that the fines were for “those that have breached the regulations” but said the prime minister had since overhauled the Number 10 operation.