NICOLA Sturgeon is under intense pressure to abandon her Valentine’s Day deadline to seal a deal on new tax powers for Holyrood after David Cameron intervened to brand it “artificial”.

The First Minister has insisted an effective cut-off point for the talks of Friday February 12 is necessary to allow MSPs adequate time to scrutinise the details of the hoped-for agreement on more fiscal powers before the Scottish Parliament is dissolved on March 23 ahead of Holyrood elections in May.

But critics have pointed out this is a period of 40 days and that, if Holyrood wanted to, it could examine and agree the framework within 48 hours.

The creation of the deadline has led some to suggest the SNP Government is intent on collapsing the whole process to spark a “blame game” with Westminster in a bid to gain electoral advantage at the Holyrood poll; an allegation strenuously denied by Ms Sturgeon, who insists her government is “busting a gut” to get a deal.

On Monday, Greg Hands, the Chief Secretary to the Treasury, will be in Edinburgh for his ninth meeting with John Swinney, the Deputy First Minister.

He has made clear he remains optimistic that a deal can be done, striking a contrasting tone to the pessimistic one exuded by the Scottish Finance Secretary, who has repeatedly made clear there remains a long way to go with time running short.

Speaking ahead of Monday’s meeting of the Joint Exchequer Committee, a spokesman for the Prime Minister stressed how “good progress” had been made to date but noted: “The First Minister set out a desire to have this done by Valentine's Day. We have always said 'let's get this done as soon as we can'.

"If we can do it by mid-February, let's do it by mid-February but, ultimately, it is an artificial deadline," he added.

A deal was due to be agreed in the autumn but the talks have become deadlocked with the impasse centred on how to reduce the £30 billion annual block grant to Scotland as Holyrood takes on enhanced tax and welfare powers.

Three options are on the table. Edinburgh favours the “per capita indexed deduction” method, which it believes best secures future funding for Scotland in light of the prediction that the UK’s population will grow faster than Scotland’s in the years ahead.

Two other options, indexed deduction and levels deduction, are unacceptable to the Scottish Government as it believes they would leave Scotland out of pocket over 10 years by £3.5bn and £7bn respectively.

London, however, views things differently. It opposes the per capita option and favours levels deduction as the latter “best answers” the Smith Commission recommendations.

David Mundell, the Scottish Secretary, dismissed as “ludicrous” the Scottish Government’s position, saying: “I don’t blame John Swinney for chancing his arm that we keep the Barnett Formula, we don’t bear any risk and, by the way, if we get any extra money, we will keep it. And if there is any extra money in England we will have a bit of that too.

“I would describe that as having your cake and eating it and then having a bit of everybody else’s cake too,” said Mr Mundell.

Failure to strike a deal would create a constitutional crisis, sparking recrimination and rancour in the run-in to the May poll.