Scottish Labour leader Anas Sarwar says he “feels sorry” for Douglas Ross in the wake of the partygate scandal.
The Scottish Conservative leader previously submitted a letter of no confidence in Boris Johnson over the row but withdrew it after Russia invaded Ukraine.
The Met Police fined the prime minister, his wife and Chancellor Rishi Sunak for breaching lockdown rules on Tuesday but Mr Ross has stood by his position.
Speaking while out campaigning in Perth on Wednesday, Mr Sarwar described the stance as “humiliating” and said its shows Mr Ross is “uncomfortable” with the actions of his own party.
‘He is clearly out of his depth’
Mr Sarwar said: “It is humiliating – I feel sorry for him.
“He is clearly out of his depth, clearly doesn’t have the respect of his colleagues, and he is clearly uncomfortable.
“If he has any backbone he should have stayed true to his principles and called for him to resign.”
After Number 10 confirmed the prime minister had been fined, Mr Ross released a statement saying it is not the right time for him to step down because of the ongoing war in Ukraine.
Mr Ross said: “As I’ve made very clear, in the middle of war in Europe, when Vladimir Putin is committing war crimes and the UK is Ukraine’s biggest ally, as President Zelensky said at the weekend, it wouldn’t be right to remove the prime minister at this time.
“It would destabilise the UK Government when we need to be united in the face of Russian aggression and the murdering of innocent Ukrainians.”
Mr Sarwar said using the war in Ukraine as an excuse for Mr Johnson saying in Number 10 is “disgusting”.
He added: “To stop democracy at home because we are defending democracy in Ukraine is a ludicrous suggestion.
“We changed prime ministers during the Second World War and during the Afghanistan War.
“Vladimir Putin probably looks at Boris Johnson and says ‘there is a clown I can contrast against’.
“The comparison is disgusting and Douglas Ross is better than that.”
Johnson must resign as PM
Mr Sarwar also reiterated his plea for Mr Johnson to step down as prime minister.
He said: “Every single day Boris Johnson remains in office he further degrades the position of prime minister.
“Both he and Rishi Sunak broke the law whilst families going through the hardest time of their life, not being able to attend funerals or be at bedsides in hospital, we had the prime minister of this country and his staff having boozy parties in Downing Street.
“The sooner they recognise they are a liability to this country and the UK deserves better, the sooner they resign.
“But I think they are shameless enough to cling on.
“But one thing Boris Johnson is, is a great unifier – he has unified England, Scotland, Wales and Northern Ireland, yes and no, leave and remain, in their disgust in their behaviour and their desire to have him booted out of Westminster.”