Gordon Brown has launched a major attack on David Cameron, accusing the Conservative leader of putting the future of the Union at risk and calling for 100,000 Scots to sign a petition against his plans.

Less than a fortnight after the former Labour Prime Minister was credited with helping to save Mr Cameron's job, Mr Brown urged the people of Scotland to band together against proposals to restrict Scottish MPs' voting rights.

He criticised Mr Cameron yesterday for that announcement, which the Prime Minister made in Downing Street made an hour after the independence referendum result.

Mr Brown also warned voters not to fall into a "Tory trap" of devolving all of income tax to Scotland. The move could destroy one of the key foundations of the UK, the pooling and sharing of resources, he warned.

It would also make the issue "England only" at Westminster, potentially barring Scots MPs votes and leaving a future Labour Chancellor struggling to pass his or her Budget.

Mr Brown's intervention signals a major rift in the pro-Union parties over the issue of extra powers for Scotland and their relationship to "English votes for English laws".

First Minister Alex Salmond said the comments were "astonishing."

It comes amid deep unease within Scottish Labour over the 100 per cent income tax push.

Some Labour politicians have accused the Tories of betrayal and attempted gerrymandering over "English votes".

Mr Brown urged 100,000 people to sign a petition to force the Prime Minister to deliver his pledge on extra powers for Holyrood "no strings attached'.

And he called for the focus to be on 14 "practical" new powers for Scotland. Among his suggestions are that up £4 billion of VAT revenues could be assigned to the Scottish Parliament. These include powers on investment, job creation, welfare, transport, elections and taxation.

He said that as part of its proposals, Labour would ensure the majority of basic rate income tax and variations in top rate income taxes in Scotland would be under the control of the Scottish Parliament.

In a direct attack on Mr Cameron, he said: "Immediately after the referendum result, a new proposal that was never raised in the pre-referendum discussions and … should have been raised before the vote, was introduced - to lower the status of Scottish MPs in the UK when voting on matters including tax."

He added that the vow "contained no ifs, no buts and had no conditional clauses and no strings attached and it was not presented as part of wider proposals yet to be unveiled, but as stand­alone and self-contained."

"In my view a vow, once written, cannot be casually re-written or revised."

He added: "The Tory trap we are in danger of falling into is to devolve all decisions on Scotland's income tax rates away from Westminster and then to deny Scotland representation in votes on Budget decisions on income tax rates.

"This would be clearly against the material interests of the people of Scotland and put the Union itself at risk."

And he called on Mr Cameron to demonstrate it is not true that on the morning after the referendum he "stopped thinking about Scotland and started thinking only about the Conservative Party."

Mr Salmond said: "This is an astonishing development. How can Gordon Brown call for people to sign a petition urging Westminster to keep its promises on more powers for Scotland when he himself has already said that is a vow that will be honoured?

"He is now calling for guarantees on the delivery of something which he said during the referendum campaign was already a done deal.

"The 'Tory trap' is not the proposals on income tax which Gordon Brown talks about - it is the Tory trap which he and his colleagues are leading people into, in which the issue of more powers for Scotland becomes entangled in a row between factions of the Westminster establishment."