LABOUR MPs predict the SNP will dramatically pull out of the Smith Commission in a fit of political pique, claiming what the Unionist parties want to agree to will not be enough.

The Smith Commission is due to publish its agreed prospectus for more powers on Thursday in Edinburgh. The crunch issue is whether or not the political parties can agree on handing over all or most income tax powers to Holyrood.

Ex-Prime Minister Gordon Brown has warned full fiscal autonomy would go counter to the pooling and sharing principle the Unionist campaign successfully promoted in the referendum campaign and would lead to the break-up of Britain.

One seasoned Labour backbencher argued the SNP "dilemma" was if there was a cross-party agreement on Smith - as the party leaderships expect - how would the Nationalists differentiate themselves from Labour in the run-up to the May 7 General Election?

He said: "Vote Labour - get Smith. So the Nationalists will have to say - Smith is inadequate. Vote SNP and we will toughen up Smith; we will not accept Smith."

The MP added he was convinced "for political reasons" the SNP would "have to break (from the Commission)".

Another senior Labour figure said: "Why wouldn't they pull out? They have been complaining for weeks. Their whole tone has been disruptive."

A third MP insisted that while the Nationalists might not withdraw next week, they would find an excuse as the process continued into next year and do so ahead of the General Election.

"They will do it for purely electoral reasons," he said.

However, one well-placed Labour insider disagreed, saying it would be more problematical for the Nationalists to withdraw.

"They'll bank what they can get but then campaign for more powers in the run-in to the election," he added.

Last night, SNP leader at Westminster Angus Robertson said: "The SNP will continue to play a productive role in the Smith Commiss-ion in good faith and work to ensure that it lives up to the expectations of people in Scotland."

A spokesman for Lord Smith said: "The talks continue to be constructive and conducted in a good spirit.

"We expect to reach an agreement involving all the parties."