Boris Johnson is not taken seriously by politicians abroad, Conservative MP Dominic Grieve has claimed.
Mr Johnson is one of the two candidates fighting to be the next leader of the Conservative Party.
Mr Grieve, who is supporting Mr Johnson’s adversary Jeremy Hunt, told talkRADIO’s Julia Hartley-Brewer: “[Boris Johnson] has many qualities but I don’t think he has the right qualities to be Prime Minister.
“Boris Johnson is not taken seriously abroad. His particular brand of humour - which may make people laugh here - does not make them laugh here.
“The evidence has always suggested to me that he has great difficulty coming across as a serious politician. For all those reasons, I think it would be unwise to have him as our Prime Minister.”
The leadership hopeful has faced scrutiny over the weekend after police were called to his flat over a domestic disturbance on Friday night.
Mr Johnson has refused to talk about the incident but the Metropolitan Police attended the south London address and found "there were no offences or concerns apparent to the officers and there was no cause for police action".
Mr Grieve said it was “legitimate” for the frontrunner to refuse to answer questions on the disturbance but that it was inevitable that people would make “judgements”.
“I think the question of public vs private life is a little bit irrelevant,” he said.
“Ultimately people will make judgements on other people’s character and that will cover their public face - it does not cover what goes on in private.
“But in so far as when private matters become public, it is almost inevitable that people will make judgements about that as well.”
He added: “My own judgement on his suitability to be Prime Minister is not based on this incident.”
Jeremy Hunt is the other leadership candidate
The leader will be chosen the Conservative membership via a postal vote, with the winner being announced on July 23.
Mr Grieve said the party members had “a duty” to think about which candidate they chose.
“If their choice is being influenced by a belief that only Boris Johnson can save us, because they have become so frightened of the consequences around Brexit, all I can say is that I think they are profoundly mistaken,” he added.
“I don’t think [Boris Johnson] will save the party at all. I think as he does not have policies, the hole that we have will get deeper and deeper.”