JACOB Rees-Mogg has made a “foolish” and “grotesque error of judgement” by attacking Douglas Ross, a former senior cabinet minister has said.

David Lidington, who was Theresa May’s deputy, said the comments made by the MP were “counterproductive” and harmed the union.

Speaking to Politicshome today, Mr Lidington said the remarks about the leader of the Scottish Conservatives being a “lightweight figure" were "crass”.

He said: "The comments were crass and completely counterproductive

“I have been in the position of trying to persuade Douglas Ross of things and he is someone who is very independent minded.

"If he has made up his mind, you have to persuade him with evidence that he should think again.”

READ MORE: Boris Johnson fails to defend Douglas Ross over 'lightweight' jibe

Mr Lidington said that Mr Ross deserved respect from the cabinet, adding: “As leader of the Scottish Conservatives, elected by the Scottish Conservative Party, he is entitled to respect from whoever serves in the UK cabinet. In terms of handling it and winning his support, it was utterly incompetent."

"In terms of the Union, which is supposed to be a government priority, it is only damaging.”

His comments come after the Prime Minister failed to directly support Mr Ross when challenged about the comments by Labour’s Stephen Kinnock.

Asked during PMQs if he also believed the Scottish Tory leader was a “lightweight”, Boris Johnson praised the Scottish Conservative party itself rather than addressing the criticism of its leader.

Mr Lidington continued: “It was a grotesque error of judgement on Rees-Mogg’s part and I hope that they will learn from this. I doubt it has done Douglas Ross too much harm in Scotland — it may well have boosted him.

"It really was a foolish way to deal with issue.”

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Mr Lidington, who was a Conservative MP for nearly 30 years, also said the Prime Minister had to “think very carefully” about his position, but stopped short of calling for him to resign.

He added: "They have to judge how the interest of the country will best be served, and what troubles me most is not the harm to my political party — and I’ve been a member under eight leaders and had agreements and disagreements with them all — but it’s the fact that the country is heading [into] a very difficult year

"For that, you need a Prime Minister and a Cabinet which has political authority, and which people are willing to trust.”