Former prime minister Sir John Major has said another Scottish independence referendum could take place if there was then a confirmatory ballot.

The Tory grandee warned that refusing such a vote could increase support for a split, saying the combination of Brexit and an unpopular UK Government has helped the popularity of separatism.

Scottish nationalists described Sir John’s comments as a “wake-up call” for Boris Johnson over his “undemocratic stance”.

Both options come with great risk. But the lessons of Brexit may offer a way ahead

In a speech delivered at the Middle Temple’s 2020 Lecture Series, Sir John said: “In law, the Scots require the approval of the Westminster Government before they can legally hold a new independence referendum.

“But refusing one might help the separatist case, by adding to the list of grievances the Scottish National Party exploit with such skill.

“The choice for the UK Government is either to agree the referendum can take place – or to refuse to permit it.

“Both options come with great risk.

“But the lessons of Brexit may offer a way ahead.

“The Westminster Government could agree for an independence referendum to take place, on the basis of two referenda.

“The first to vote upon the principle of negotiations, and the second upon the outcome of them.

“The purpose of the second referendum would be that Scottish electors would know what they were voting for, and be able to compare it to what they now have.

“This did not happen with Brexit: had it done so, there may have been no Brexit.

“Many Scottish voices – and especially business – may support the logic of this: it may focus minds away from a short-term reflex opposition to a perceived English Government, and back to the mutual and long-term virtues of the Union.”

Voters in Scotland chose to remain in the UK by 55% in the 2014 Scottish independence referendum.

Scottish Secretary Alister Jack has recently again ruled out another ballot taking place for at least another generation, although he would not define that timeframe.

Polls this year have shown an increase in support for separation from the UK.

Kirsten Oswald, the SNP’s Westminster deputy leader, said: “The remarks from the former Tory prime minister must serve as a wake-up call for the current Prime Minister over his undemocratic stance – straight out of the Trump playbook – to attempt to deny the results of a democratic election by trying to block the people of Scotland from having the right to choose their own future in a referendum.

“Poll after poll has shown that independence is now becoming the settled will of the majority of people in Scotland, and it is for the people of Scotland to decide their future.

“It is not for out of touch Westminster governments to dictate the terms of a referendum or to dictate the future of the people of Scotland.”