Scottish Conservative leader Douglas Ross has come under attack for dodging a key Commons vote over whether the Prime Minister should face investigation over the partygate scandal

MPs are due to vote on Thursday on whether the Commons Privileges Committee should carry out a probe to determine if Boris Johnson misled Parliament with his denials on the issue.

Reports suggest Mr Ross – who is both an MP and an MSP – will not be in Westminster for the vote, prompting criticism from his political rivals.

Boris Johnson is facing a key Commons vote on Thursday (Victoria Jones/PA)

The attacks came as a former senior Scottish Tory warned the party could pay the price at the ballot box for its support for the Prime Minister.

Mr Ross had previously called for Mr Johnson to quit over lockdown parties in Westminster, but the Scottish Conservative leader later withdrew a letter of no confidence in the Prime Minister, saying the conflict in Ukraine made it important for him to remain in charge at Downing Street.

That comes despite the Prime Minister having to pay a fixed penalty notice for breaching his own Covid rules.

SNP Westminster leader Ian Blackford insisted Mr Ross should be in the Commons for Thursday’s vote to ensure Mr Johnson “is held to account properly by the Privileges Committee”.

The SNP MP told BBC Radio Scotland’s Good Morning Scotland programme that “Douglas Ross has been all over the place on this”.

He added: “He seems to think the Prime Minister has got questions to answer, but he’s not prepared to vote tomorrow to make sure that this Prime Minister is held to account.

“When you have got someone who behaves in this way, is cavalier in his approach to office, someone who is not fit for purpose to be prime minister, Douglas Ross should be there, making sure that this man is held to account properly by the Privileges Committee.”

Mr Blackford repeated his calls for Mr Johnson to quit, saying there “is a very clear case now that the Prime Minister has not just broken the law, that he has misled Parliament”.

He added: “What you have with this Prime Minister is a sense that the rules don’t apply to him, that he is above the law, that he can do as he pleases.

“He treats Number 10, he treats the office of being Prime Minister, as a personal plaything, and for him this is over.”

Scottish Liberal Democrat MSP Alex Cole-Hamilton told BBC Radio Scotland that Mr Johnson had presided over a “culture of repeated rule breaking” at a time when others were expected to follow strict lockdown rules.

Mr Cole-Hamilton said: “Douglas Ross has a lot to answer for here. He wants the country to believe he is the person to stand up to the SNP and to Nicola Sturgeon but he can’t even stand up to his own boss in London.”

Adam Tomkins, who was on the Scottish Conservative frontbenches at Holyrood before quitting as an MSP last May, said he expected voters to deliver a “withering” verdict on the Tories.

Prof Tomkins branded Mr Johnson a “fool and a clown” as he insisted the Prime Minister “must be shown the door”.

Former Tory MSP Adam Tomkins (Jane Barlow/PA)

He also warned Tories in Scotland that having taken the “high road of principle earlier in the year”, the party’s stance on the matter now looked “not only empty but risible”.

Writing in The Herald newspaper ahead of May’s local government elections, the former MSP said: “I expect that the popular verdict on this pantomime of a performance will be every bit as withering as it deserves to be.

“There are some good, talented and principled men and women in the Scottish Conservative party but, yet again, the vehicle in which they insist on remaining is leading them badly astray.”

He said Scottish Labour could overtake the Tories to become “Scotland’s second party and the principal opposition force to the SNP”.

Prof Tomkins, a law professor at Glasgow University, added: “I fully understand that loyalty is the currency of politics. But there are higher values than loyalty. Fidelity to legal principle is one of them.

“A rule-breaking government cannot be supported. And a rule-breaking prime minister is no ruler.”