UNIONISTS have launched a campaign to raise the bar for a second independence referendum by insisting any constitutional change must be backed by two-thirds of voters.

The Scotland Matters group, which includes figures from the Better Together campaign of 2014, has lodged a petition at the Scottish Parliament on the issue.

It coincides with the Scottish Government advancing its Referendums Bill through Holyrood in order to pave the way for a second independence referendum in late 2020.

Nicola Sturgeon has said she will ask the UK Government to transfer vital referendum powers it has so withheld by January next year.

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The Scotland Matters petition, which is cited in a report to Holyrood’s constitution committee tomorrow about the Referendum Bill, says a two-thirds vote for change would avoid a disputed result.

Rather, it would be “clear, demand acceptance by all, and neutralise subsequent efforts to revoke the result of have another referendum” and give a “powerful mandate” for the government implementing the change.

It goes on: “The narrow Brexit vote and ongoing difficulty of a British Government to negotiate and implement a settlement is an example of the problems resulting from deciding an issue on anything less than a substantial majority that gives a widely accepted mandate.

“Very few people questioned the 75/25 per cent result of the 1997 Scottish devolution referendum.”

The vote to establish the Scottish Parliament won 74.3% of the vote, while the second question on giving the parliament tax-raising powers was backed by 63.4%.

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Two-thirds majorities are not unusual in constitutional matters.

A two-thirds majority of MSPs is required to change Holyrood’s electoral system, and the support of two-thirds of MPs is needed to hold an early election at Westminster under the Fixed-Term Parliaments Act.

The SNP also uses a two-thirds majority to change its own party constitution.

However thresholds of more than 50% have been highly controversial since the 1979 referendum on devolution when most people who voted backed a Scottish Assembly, but the No side won because the Yes vote was below 40% of the entire electorate.

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Mark Openshaw, of Scotland Matters, said: “With the division and chaos caused by recent referendums, it’s time for our MSP’s to set the bar high enough for everyone to accept a result. Recent events prove the 50 per cent plus one threshold is not enough.

“A two-thirds majority would put the result beyond doubt. There are precedents for such a system: the SNP itself needs a two-third benchmark for changes to its party constitution.”

Psephologist Professor Sir John Curtice dismissed the idea of a threshold last month, telling an Edinburgh Fringe audience that any constitutional position opposed by a simple majority of people was unsustainable.