A NEW Act of Union to create a federal United Kingdom is the last chance to save the 300-year-old accord and stop the drive towards Scottish independence, a cross-party campaign group has insisted.

Former Northern Ireland MP David Burnside, one of the Constitution Reform Group’s (CRG) leading lights, said the biggest threat to the union remained Scottish independence.

Insisting the constitutional nettle had to be grasped before Nicola Sturgeon called a second independence referendum, Mr Burnside said: “55-45 was meant to be a great victory for Unionism. In Scotland, the interpretation was the SNP almost did it.

“If there is a decent In vote, David Cameron stays as prime minister. David is a Unionist. His mind can be open on this one. There is a window of opportunity to persuade the Conservative leader and the Labour Party, which got a real hammering in the election, the way to return is to have a positive campaign against the SNP’s positive campaign; that there are positive advantages to being British and Scottish, British and English, British and Welsh, British and Northern Irish.”

Recently, leading CRG members, who included Mr Burnside, the Marquess of Salisbury, the former Conservative Leader in the Lords, and Gisela Stuart, the senior Labour backbencher, held a private meeting at No 10 with the Prime Minister.

He said: “His stated position is that we don’t need this at the minute; he believes standing orders in the Commons will deal with the problem. But he opened the door to the extent that he has asked us to start liaising with the Cabinet Office.”

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Asked if he regarded the CRG’s three-year window of opportunity as the last chance to save the Union, the ex-MP for South Antrim replied: “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.”

The draft bill is in its final stages with financial experts poring over the details. The campaign will be launched in July in the four parts of the UK.

Mr Burnside said that his “tribe” was as an Ulster Unionist and Orangeman but insisted support for the Union was wide and even existed among some SNP supporters.

He said: “I was very happy to see John Reid, the former chairman of Celtic, campaigning for the Union. So this is one campaign where you can be a Rangers supporter or a Celtic supporter, or a Hibernian or a Hearts supporter and be for the Union. And you can have all your Scottish traditional loyalties, whatever flag or scarf you want to wear or not wear and still be for the Union.”

The former MP insisted that the idea of creating a federal UK had now to “get into the mainstream of Westminster and Whitehall”.

The basis of the CRG’s campaign – whose members also include Lord Campbell, the former Liberal Democrat leader, and Lord Hain, the former Labour Northern Ireland Secretary – is to introduce a Bill to the UK Parliament to create a ‘confederal’ system, in which Scotland, England, Wales and Northern Ireland decide which powers they wish to retain and which they want to hand to the centre at Westminster and Whitehall.

Under the proposals, the House of Commons would become the English parliament and the House of Lords the UK federal parliament, to which each part of the UK would send members.