Boris Johnson could be the UK’s last prime minister, Gordon Brown has warned, after the Conservative leadership frontrunner met Scottish MPs to reassure them of his desire to bolster the union.

Johnson’s meeting with 11 out of the 13 Scottish Conservatives at Westminster on Wednesday is understood to have covered when he should first visit Scotland if elected, how colleagues north of the border could feed into policy decisions and the future structuring of government – Johnson has proposed a “union unit” within Downing Street. The meeting was described as “positive” and “businesslike”.

Brown, the former Labour prime minister, warned of an approaching “head-on conflict” between Johnson’s hardline views and the Scottish National party’s “extreme nationalism”, with its intention to abandon the pound and leave the UK single market and customs union.

Boris Johnson's no-deal Brexit threat risks being ignored by the EU Read more

Writing in the Scottish Daily Mail about his plans for a new thinktank to make the “positive, patriotic and progressive case for the union”, Brown said: “Nothing illustrates the sterility of this head-to-head confrontation better than yesterday when, in the wake of news of Scotland having the worst and most deadly drug problem in Europe, the SNP and the Conservatives simply blamed each other.”

Brown said Johnson’s brand of anti-Europe conservatism was regarded in Scotland as anti-Scottish and that “no matter what he may say now, two decades of anti-Scottish invective will come back to haunt him.”

After Theresa May used her final visit to Scotland as prime minister to warn her successor of the threat posed by a no-deal Brexit to the integrity of the UK, both Johnson and his leadership rival, Jeremy Hunt, insisted at a hustings in Perth that the union would come first if there was a tension between delivering Brexit and the unity of the UK.

Since then, references to the future of the union have been notably absent from other hustings and candidates’ events.

Johnson’s relationship with the Scottish Conservative leader, Ruth Davidson, who has backed Hunt after initially supporting Sajid Javid and then Michael Gove, is known to be strained. Scottish Conservative MSPs are privately concerned about how Johnson would be received on doorsteps in the next Holyrood election campaign.