BORIS Johnson once said that a Scottish MP should never be prime minister as “government by a Scot is just not conceivable”.

Writing in The Spectator in 2005, the former foreign secretary, who looks set to be the next leader of the Tories, said it would be “utterly outrageous” if Gordon Brown succeeded Tony Blair to become prime minister.

“The Labour machine will try, at some point in the next few years, to insert Gordon Brown,” Johnson wrote. “That would be utterly outrageous, not just because he is a gloomadon-popping, interfering, high-taxing complicator of life, but mainly because he is a Scot, and government by a Scot is just not conceivable in the current constitutional context.”

READ MORE: Scottish Tory MPs reveal support for Boris Johnson and Michael Gove

In the comments, unearthed by The Scotsman, Johnson claimed that Brown was “not really interested in British values” but instead in “his personal political disability as a Scottish MP.”

Responding to the comments on Twitter, Nicola Sturgeon said Johnson would “probably find we think the same about him”.

It’s the second time in less than a week that old comments about Scotland and the Scots have come back to haunt the Eton-educated MP.

Last week, a poem – published in the Spectator while Johnson was editor – describing Scottish people as vermin who should face “extermin-ation” was re-unearthed.

It describes Scots as “tartan dwarves” who were “polluting our stock” and suggests that the country should be turned into a “ghetto” with the inhabitants submitted for “extermination”.

Meanwhile, in an excruciating moment for the Johnson campaign, one of his key allies was left stumped after a journalist asked how many children the ex-mayor of London has.

Radio 5 Live presenter Emma Barnett was quizzing Johnny Mercer MP about his support for Johnson.

She then asked about a privacy judgement from 2013 that revealed that Johnson had a child with a woman who was not his wife.

The child’s family sought an injunction to stop the father being named – but lost as judges ruled that the public had a right to know about Johnson’s behaviour as he was in charge of London at the time.

“How many children does Boris Johnson have?” Barnett asked.

Mercer replied: “I’m really not interested in Boris Johnson’s family life.”

Barnett said: “Because right now if you google ‘Boris Johnson, how many children does he have’ – and I’ll explain why I am asking this – it comes up as four.

“That is with his still wife, but now separated, Marina Wheeler.

“But there was a court case in 2013 where a woman lost a privacy case.

“The judge said it was in the public interest while he was in office that he had an extramarital affair with a woman who later gave birth to a daughter while he was married. It then came out in that court case that it was alleged he had another child out of wedlock.

“So it is between five and six children that our future prime minister has.

“Would you not say it is fair and right to get an answer to the question of how many children our next prime minister has?”

Mercer replied: “Discussing our private lives is a decision that public figures like me make.

“I have chosen to discuss mine, Boris has not chosen not to discuss his and he is well within his rights to do that.” Barnett then asked Mercer why Johnson had chosen “not to be honest about something as basic as how many children he has?”. Mercer said it would inappropriate for him to talk about any of that.

Tory MPs will vote in the second ballot of the party’s leadership today.