A BID to make it a requirement for at least two-thirds of voters to support independence in a new plebiscite before a Yes victory could be declared would breach a code of good practice on referendums drawn up by advisers to Europe’s leading human rights organisation.

The revelation emerged after a pro-Union group 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.

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Scotland Matters lodged a petition at Holyrood which has been cited in a report to the Scottish Parliament’s Constitution Committee today about the Referendums Bill, arguing a two-thirds vote for change would avoid a disputed result.

However, The National can reveal the most recent guidance on referendums by the Venice Commission, which is an advisory body to the Council of Europe and made up of constitutional experts, opposed the use of such thresholds.

In its Code of Good Practice, published last October, it states: “It is advisable not to provide for: a) a turn-out quorum (threshold, minimum percentage), because it assimilates voters who abstain to those who vote no; b) an approval quorum (approval by a minimum percentage of registered voters), since it risks involving a difficult political situation if the draft is adopted by a simple majority lower than the necessary threshold.”

It goes on to say a “turnout quorum” – ie a threshold requiring a minimum turnout of voters – would mean that it would be in the interests of those opposed to a proposal to simply not vote. The code says this would “not be healthy for democracy”.

The report added that “an approval quorum” – acceptance by a minimum percentage of registered voters – “may also be inconclusive”.

The code states: “It may be so high as to make change excessively difficult. If a text is approved – even by a substantial margin – by a majority of voters without the quorum being reached, the political situation becomes extremely awkward, as the

majority will feel that they have been deprived of victory without an adequate reason; the risk of the turnout rate being falsified is the same as for a turnout quorum.”

The Council of Europe is a human rights organisation consisting of 47 members including the UK.

Nicola Sturgeon plans to hold a second independence referendum late next year and has said she will ask the UK Government to transfer referendum powers it has so far withheld by January next year.

The Scotland Matters petition says a two-thirds vote for change would avoid a disputed result and yield a “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.

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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.”

The 1979 referendum on Scottish devolution set a requirement that 40% of the total electorate would have to back devolution for the result to be valid. As it turned out 51.6% of voters voted Yes, but because turnout was 63.6%, it meant that only 32.8% of the electorate had voted for devolution. Devolution was therefore rejected in 1979 and the Scotland Act 1978 was repealed. The result caused controversy in Scotland and the “40%” rule was seen as unfair.