LABOUR will today call on David Cameron and Nicola Sturgeon to ditch their “arbitrary” Valentine’s Day deadline for a deal to introduce Holyrood’s new tax powers as Treasury chief Greg Hands faces a grilling by MPs.

The Tory Cabinet Minister is to be questioned over the deadlock at the SNP chaired Commons Scottish Affairs Committee shortly before Labour’s Ian Murray rises at the dispatch box to lead the first debate on the fiscal framework, the mechanism to be used to introduce the Scottish Parliament’s new tax and welfare powers.

The Shadow Scottish Secretary will call on both governments to provide full transparency on the negotiations, drop the “blame game” and concentrate all their efforts on securing a deal.

With time fast running out, failure to reach an agreement would lead to rancour and recrimination in the run-in to the main campaign for the May 5 Scottish parliamentary elections. It would also throw the Scotland Bill into constitutional limbo with the powers delayed for at least a year until 2018.

“A deal must be done,” declared Mr Murray ahead of today’s debate. “People across Scotland want politicians to do their job and find an agreement that works so that we can get on with using these powers. Neither the UK nor Scottish Government negotiators should walk away until we have the new powers secured.”

The Edinburgh MP insisted Scots would “not understand that after the negotiation for more powers, this deal could fall apart at the last minute”.

Elsewhere at Westminster, Mr Hands will be cross-examined on the delay that has dogged the negotiations; both governments initially said an agreement would be reached last autumn.

“The clock is ticking in securing a deal on the fiscal framework and we need to probe the Chief Secretary to the Treasury on why there seems to be an impasse,” said Mr Wishart.

“This is the first time that the Government has given direct evidence to a parliamentary committee and it is important we get a sense of what they see as the key issues. With a deadline of February 12 close at hand, we will be seeking assurances from Mr Hands that a deal will be reached in time,” added the Perth MP.

The nub of the problem is how to reduce the £30 billion annual block grant as more powers are introduced at Holyrood. The context is the Smith Commission’s principal recommendation that whatever mechanism is used, there should be “no detriment” to either Scotland or the UK.

John Swinney, the Deputy First Minister, leading for the SNP government prefers one option, "per capita indexed deduction", which he believes will safeguard Scotland’s revenue in the years ahead. But Mr Hands, leading for the UK Government, prefers another option, that will prevent UK taxpayers paying over the odds to help fund Scotland’s public services.

After eight meetings, with a ninth pencilled in for next week, the two parties remain at loggerheads.

In a separate development, the STUC swung behind Mr Swinney’s position with Grahame Smith, its General Secretary, saying anything other than the Scottish Government’s preferred option “would be to the severe detriment of future public spending in Scotland”.