RESTRICTING Scottish MPs' voting rights at Westminster because of increased tax powers for Holyrood would lead to the destruction of the United Kingdom, Gordon Brown has warned MPs.

His stark message against creating two classes of MP was delivered in the face of Conservative backbenchers demanding "fairness for England" in a Commons debate on devolution yesterday.

William Hague, the Leader of the House, cautioned that failure to address the legitimate concerns of England on devolution would be an even "greater danger" to the Union.

Condemning the Tory proposal to devolve all income tax power to Holyrood, which would lead to calls for Scottish MPs to be banned from financial votes like the Budget, the Kirkcaldy MP said: "You can't have representatives elected by the people who are half-in, half-out of the law-making process.

"Let us remember the words of the New Testament in Mark quoted by Lincoln - 'a house divided cannot stand and a house divided is wrought into desolation'. That is the truth of what the Conservative Party are now proposing."

During a heated parliamentary debate, the Nationalists complained that none of the UK party leaders who had signed the famous 'vow' could be bothered to attend while, earlier, First Minister Alex Salmond raised the possibility of a second referendum should the promises made on more powers for Holyrood not be delivered.

As MPs prepared for the devolution debate, Labour announced it would not take part in talks about English votes for English laws (EVEL), which would lead to restricting Scottish MPs' voting rights, branding the process a "No 10 stitch-up". Ed Miliband has called for a constitutional convention to consider the matter in depth.

In response, David Cameron said: "What we need is obviously more devolution for Scotland but a settlement that's fair for the whole of the United Kingdom."

The Prime Minister added that the boycott was a matter of "great regret" but it showed the Labour leader and his colleagues were "not interested in fairness" for England.

During the six-hour debate, several Conservative MPs argued for EVEL, insisting it was only fair, that in light of more powers promised for Holyrood, Scottish MPs should be prohibited from voting on England-only matters.

Mr Hague pledged there would be no dither and delay on delivering the promised powers for Scotland but warned "insensitivity and indifference" to concerns over the English votes issue risked the future of the UK.

However, Mr Brown attacked Mr Cameron's proposal on EVEL as well as the Tory policy to devolve all income tax power to Holyrood.

"It's the combination of the two proposals to devolve 100 per cent of income tax and then to remove the right of Scottish MPs to vote on the matter in Westminster that is absolutely lethal to the constitution," declared the ex-PM, who insisted there could not be one United Kingdom if there were two classes of MP.

He insisted the English Question could be addressed without disrupting the status of Scottish MPs and while simultaneously "meeting the sensitivities of the English".

"There are other ways that we could meet the needs of English MPs without creating two classes of representation in this House, by which time the Union would be all but over," Mr Brown added.

Earlier, the First Minister accused the UK parties of what looked like a "preparation for a betrayal of a strong commitment made" on more powers and said that while he felt a referendum was a one in a 20-year event, circumstances could change.

"Clearly, if you had a situation where three leaders made such a public vow...and then reneged upon it, then that would obviously be a very, very substantial change of circumstances."

But he added that it would ultimately be for Scottish voters to decide whether they had been "conned and tricked by Westminster leaders".

Mr Hague suggested Mr Salmond was "almost looking for and hoping for some sense of betrayal" but insisted there would be none as the UK parties were completely commitment to honouring their pledge.