UK needs federal system to avoid “constitutional instability” of Scotland Act and Evel, says expert

“Furthermore, the devolution of tax-raising powers is opaque and accountability of the Scottish government to the Scottish people will be weak.”

Read more: Federal UK is ‘last chance to save the Union’

Prof Booth added: "We need a fully federal system with power being exercised by the UK government only in areas such as defence and border control. Other issues would then clearly be the responsibility of the individual nations within the UK. Accountability would be very clear and the settlement stable."

His remarks come as the campaign group, the Constitution Reform Group, gears up for a summer launch to create a new Act of Union to change the UK into a federal structure, where each of the four parts control all power but then decide to give power back to the centre at Whitehall and Westminster.

Under this arrangement, the House of Commons would become the parliament for England while the House of Lords would become the federal parliament with members drawn from Scotland, England, Wales and Northern Ireland. The expectation would be that the federal parliament would oversee defence and foreign policy as well as general taxation.

A draft bill is currently being drawn up with the campaign launch pencilled in for July. If the legislation were ever to pass successfully through Westminster, it would, before getting royal assent, be put to people in a referendum. Each of the four parts of the UK would have to back the proposal or it would fall.

Read more: New Scotland Act powers make a case for creating a Scottish chancellor, says John Swinney

David Burnside, the former Northern Irish MP, is a leading light within the CRG, and believes creating a new Act of Union is the only way to stop the drive towards Scottish independence by creating a federal system. He says the group, which includes peers like Lord Campbell, the former Liberal Democrat leader, Lord Salisbury, the ex-Tory leader of the Lords, and Peter Hain, the former Labour Northern Ireland Secretary, has a “window of opportunity” of a few years before the SNP leadership seeks another independence referendum.

Asked if he regarded the CRG's bid to create a federal UK as the last chance to save the Union, Mr Burnside said: "Yes, because the SNP have got a professional machine. Anyone who can keep up this level of political support when their economic and fiscal policies are in meltdown - and not just on oil prices - anyone else would be taken apart. So they have a momentum as a professional machine, which people should respect and understand."

Read more: How, exactly, can these Evel plans work?

Meantime, Daniel Greenberg, a leading constitutional expert has hit out at the speed with which Scotland's sweeping new tax and welfare powers were passed by MPs.

As parliamentary counsel for two decades, he drafted Acts in all areas of law, including tax, immigration, housing and constitutional law, including Northern Irish devolution.

But Mr Greenberg claimed the Commons had considered "highly technical” legislation incredibly quickly while a number of large-scale changes were voted through without even being debated.

He suggested most people, whatever their views on devolution, would “be both surprised and alarmed to hear that a Bill fundamentally altering our constitutional arrangements passed through the House of Commons at the overall rate of about half an hour of consideration per page of detailed, highly technical legislation; with many provisions in fact not being considered at all".

Mr Greenberg added: “The accelerated increase of legislation of all kinds means that politicians from all parties need to think more about how to make traditional legislative scrutiny methods fit for purpose in the modern world."