A second independence referendum should not be held for at least 15 years, according to a group of devolution experts.

The academics also warn that the SNP's call for 'devo max', or full fiscal autonomy, would break up the Union, in a new report published today.

Called A Constitutional Crossroads: Ways Forward for the United Kingdom it calls for the abolition of the Barnett formula, which helps to calculate the block grant to Scotland, because it does not offer a fair solution across the UK. .

The group also recommend a new 'charter of the Union' to protect the UK, English votes for English laws at Westminster and greater decentralisation within England.

Last night the SNP said that a second referendum on independence would be a matter for future Holyrood elections and the Scottish people.

Last week SNP sources suggested that Scottish ministers did not need David Cameron's permission to hold another vote.

Technically, the legal right to stage a referendum lies with Westminster.

In recent days the Prime Minister has outlined his opposition to another vote.

But the First Minister Nicola Sturgeon has said he has no right to rule one out.

The report, compiled by experts including Professor John Kay, a former adviser to Alex Salmond, Prof Sir Jeffrey Jowell, director of the Bingham Centre for the Rule of Law, Prof Linda Colley, from Princeton University, Prof Adam Tomkins, from Glasgow University, Prof Tony Travers, from the London School of Economics and devolution expert Alan Trench, suggests a Scottish independence referendum be held "no more than once in a generation.... (and) a generation should be considered at least 15 years."

On devo max, the SNP's preferred option, it warns: "Such an extreme form of devolution would seem designed not to preserve the Union with the rest of the United Kingdom, but to break it."

The report recommends a new 'charter of the Union', enforceable by the courts.

The present Barnett formula governing funding arrangements for the devolved governments "does not deliver equity between the various parts of the UK" it concludes.

"The way forward is a grant mechanism based on clear criteria such as relative need with a discount for devolved taxraising powers and subject to periodic reviews," it adds.

Earlier this week a Royal Society of Edinburgh working group called for an overhaul of the Barnett Formula, despite fears such a move could see Scotland lose billions of pounds.

SNP Justice and Home Affairs spokesman Joanna Cherry said: "Westminster needs to deliver the additional powers which Scotland has been promised - and the recent election result is a huge mandate for further powers beyond those recommended by the Smith Commission.

"In terms of a second independence referendum, that is a matter for future Holyrood elections, and whether or not there will be one is ultimately a matter for the people of Scotland."