DAVID Cameron has spoken personally to Nicola Sturgeon in a bid to break the deadlock over tax powers as Labour today urges both leaders to “get round the table” and seal the deal.

The Herald has been told the respective leaders– who spoke on the telephone on Monday - share a determination to make the talks work but each recognise there is “more hard work to do” to overcome the impasse.

The talks continue to stall over the what so-called no detriment rule means in practice.

While more discussions between John Swinney, the Deputy First Minister, and Greg Hands, the Chief Secretary to the Treasury, could take place in the coming days, it now seems certain that an agreement will not be reached by Valentine’s Day as Ms Sturgeon had hoped.

Speaking to reporters at Bute House in Edinburgh, she vowed to explain how she would use Holyrood's new powers regardless of whether a deal was done to approve the Scotland Bill before the Scottish parliamentary elections on May 5.

The FM tore up her previous February 12 deadline, saying she would do "everything in my power" to get a deal that MSPs could approve in time for the Holyrood poll.

She said: "I want these additional powers; I've spent my entire adult life campaigning for a Scottish Parliament that is as powerful as possible.

"Let me be clear,” Ms Sturgeon added, “whether or not a deal is struck before the election - and I hope a deal is struck before the election - my manifesto will set out how we would choose to use these powers. But for any party to use them meaningfully, we need a fiscal framework that is fair."

Following the telephone call, Ms Sturgeon wrote to the Prime Minister to complain that the two governments are not working on a "shared understanding" of the no detriment principle, which says at the point of further devolution neither Scotland nor the UK should be worse off as the tax powers are transferred and the annual block grant reduced.

The Scottish Government believes the Treasury’s offer would “embed” a systemic cut in funding of £3 billion over 10 years because Scotland's population is not forecast to grow as strongly as the UK’s.

The UK Government insists it has “bent over backwards” to address Holyrood’s concerns. It insists “no detriment does not mean no risk” and that UK taxpayers should not have to pay more for Scottish public services simply because the UK population will see a larger rise than Scotland’s.

Labour's Ian Murray, the Shadow Scottish Secretary, is today expected to raise the issue of a face-to-face between the leaders during Scottish Questions in the House of Commons.

He said: “Making a deal on the fiscal framework is now all that stands between the Scottish Parliament and the substantial new powers it needs to make Scotland a fairer nation.

“We have had nine ministerial negotiating meetings and still no agreement has been reached. During this time, the Prime Minister has been flying around Europe trying to strike a deal on EU reform but hasn’t got involved in the important negotiations on his own doorstep.

“A deal must be done and after six months of meetings, with so much at stake, it’s time for the Prime Minister and the First Minister to get round the table, give these negotiations the attention they deserve and seal the deal for Scotland and the UK,” insisted the Edinburgh MP.