CONSERVATIVE ministers are considering introducing a referendum act that could wreck the campaign for Scottish independence, according to reports today.

The plan would require more than half of Scotland’s entire electorate, rather than a simple majority, to vote to leave the Union before it would be allowed.

Senior government figures want to legislate quickly to introduce new barriers to independence in the event that the Supreme Court rules later this year that SNP plans to stage a referendum without UK consent are unlawful, reports say.

Eight years after the matter last went to a vote, the plan would require evidence for more than a year that at least 60 per cent of voters want a new referendum on independence before the UK government would even consider it.

Then in the event that ministers agreed to hold one, a simple majority of 50 per cent plus one of those who vote — as with the 2016 EU referendum and the 2014 independence referendum — would not suffice. Instead, at least 50 per cent of all of Scotland’s electorate would require to vote to leave the Union before the change would be passed.

In 2014, when 45 per cent voted for Scottish independence and 55 per cent voted against, 85 per cent of the electorate turned out. Had the proposed approach applied back then, the Yes side would have required to secure more than half a million extra votes to win. Last time Yes gained support from 38 per cent of the electorate and No 47 per cent while 15 per cent did not vote.

The proposed change, which has been condemned this morning by First Minister Nicola Sturgeon and triggered concerns among Tory politicians that it could prompt a pro-independence backlash, echoes a contentious amendment by the Labour MP George Cunningham that scuppered devolution for Scotland in a 1979 referendum.

Although a majority voted for devolution, it failed because of a legislative stipulation that 40 per cent of the total Scottish electorate had to endorse the plans.

A senior government ally of Liz Truss, who is expected to be named as the UK's next prime minister this week, defended the plan.

He said: “After all, the SNP said after the 2014 referendum that they would not seek another one until polls consistently showed more than 60 per cent of the public would vote to leave the UK.

“In order to achieve independence it would not be unreasonable for the Yes side to demonstrate that it was the settled will of the Scottish people like in the 1997 devolution referendum where there was a three to one majority in favour of a Scottish parliament.”

The source told the Sunday Times that the government could act after the Supreme Court gives its verdict on Ms Sturgeon’s plan to hold a referendum without UK approval.

However, there is growing concern among some other senior Tories that a more “macho” approach to the SNP under Truss that is not sufficiently respectful of other opinion north of the border will play into nationalist hands.

Some were horrified when the Conservative peer Lord Frost argued last month that three quarters of MSPs should support independence before a new independence referendum can be allowed, and suggested that some devolved powers could be returned to Westminster.

With Ms Sturgeon expected to lose the Supreme Court case, some senior unionists believe the nationalist movement could soon be severely weakened with nowhere to go, and that provoking Scottish opinion by raising the bar for independence is highly risky.

A senior Conservative who masterminded the successful 2014 campaign to save the Union and who has advised the UK government on the constitution since last year is quitting as Britain’s new prime minister takes over.

Lord McInnes of Kilwinning, a director of the Scottish Conservatives for 18 years before being hired by Boris Johnson, is standing down as a special adviser to pursue a new career.

While the departure is said to be amicable, there are concerns that it could also damage the unionist cause at a time when some believe Truss needs “cool heads” around her.