THE first major row between the pro-Union parties over extra powers for Holyrood has erupted amid accusations that David Cameron's surprise bid to restrict Scottish MPs' voting rights has made it harder to strike a deal.

Sources close to Scottish Labour leader Johann Lamont have suggested the Prime Minister's "English votes for English laws" announcement, made after the No vote in the independence referendum, makes it more difficult to reach a compromise on devolving income tax to Holyrood.

The warning is the first major sign of simmering tensions between the parties over new powers.

Ken Clarke, the former Tory Chancellor, has intervened in the row, urging UK ministers to "hold their horses" in trying to solve the so-called English Question.

"We've got to resist an English ­backlash," he declared during a fringe meeting at the Conservative conference in Birmingham. "I'm a Unionist - we're trying to hold the Union together, not encouraging the other nations to start getting stirred up."

The Tory big beast made clear the extra-powers pledge for Holyrood must be honoured within the agreed ­timetable, but added: "I would hold my horses on the rest because there is a whole separate debate to be had about devolution within England.

"There are all kinds of other issues raised about the relationship between England and the three smaller nations and the position of the Welsh, which gets ever more anomalous."

Ed Miliband has called for a public consultation on the English Question followed by a constitutional convention after the 2015 General Election.

The Tories have pledged they would push for 100 per cent of income tax to be devolved, with a senior source describing the figure as a "floor" that they would not go below.

Labour wants MSPs to be responsible for only a portion of the levy.

All three parties pledged a new deal for Holyrood as part of the fight against independence.

As his party conference opened, Mr Cameron insisted the vow on more powers would be kept and added it was separate from moves for "English votes for English laws".

Yet sources close to Ms Lamont suggested the PM's surprise announcement, less than an hour after the referendum result had come through, was disruptive. One said: "The English votes issue has made it more difficult for us to reach a compromise with the Tories. Where would it leave a Labour Chancellor if he could not rely on the votes of dozens of Scottish Labour MPs?

"If Ed Balls was the Chancellor and needed to change the rate of income tax he would no longer be able to rely on Scottish MPs to put through a Budget."

He added: "That is quite apart from the fact there are principled reasons behind devolving only a portion of income tax." And he played down claims Labour was ready to accede to Tory demands in order to get welfare powers, saying: "We have not thrown our position overboard already."

Labour argues that giving full powers

over income tax rates to the Scottish Parliament could undermine the principle of pooling and sharing resources within the UK."

William Hague, who chairs the new cabinet committee on "English votes" told delegates: "We are wholeheartedly committed to the further devolution to Scotland and Wales that is rightly on its way. But such further devolution turns the matter of Scottish MPs continuing to vote on even the most minor policies in England from an anomaly into an injustice, and from an issue that many want to postpone into one that must be now be faced. It is time for the way decisions are made to be fair to all, including the voters of England."

Tory chairman Grant Shapps said the move was a "common-sense solution for the English people".

He said it was "the politics of fairness for the 53 million people in England."

Mr Shapps accused Labour leader Ed Miliband, who has called instead for a UK-wide constitutional commission, of an "inability to put our country before your party, [which] means you will never be fit to hold the office of Prime Minister of our United Kingdom."

No 10 insists that extra powers for Scotland will happen with or without English votes.

But the English votes proposal will run in parallel with that on Scottish powers. Initial plans will be published at the same time, in January next year.

Labour claim that English votes for English laws cannot help but affect the three smaller parts of the Union, not least because of their potential financial impact.