London and Edinburgh are engaged in a tense stand-off over the mechanism to introduce Holyrood's new tax powers as the intergovernmental negotiations show no sign of success and every sign of edging towards collapse.

With time running out ahead of the Scottish Government's imposed deadline of February 12, a somewhat glum John Swinney emerged from another round of Treasury talks, expressing no admission of progress or confidence of a breakthrough to secure what Nicola Sturgeon has termed a Valentine's Day deal.

"There is still some considerable distance to travel to resolve these issues," declared the Deputy First Minister.

He explained the powers being transferred were “valuable and important powers” but stressed: “We can’t exercise those powers at any price; we’ve got to get a fiscal framework that is right for the people of Scotland, that is fair to the people of Scotland is consistent with the Smith Commission proposals.”

He added: “The people of Scotland would not forgive me if I signed up to a deal that is financially damaging to the people of Scotland and I wouldn’t do it.”

A spokesman for the UK Government insisted the talks had “shown yet again our willingness to engage with and listen to the Scottish Government’s concerns”.

He admitted that while some difficult issues still had to be resolved, the Treasury remained “confident that a deal can be reached that is fair to Scotland and fair to the rest of the UK now and in the future”.

Failure to get a deal would spark a constitutional crisis ahead of the May elections and put back the introduction of the new powers from April 2017 to April 2018.

The eighth meeting of the Joint Exchequer Committee(JEC) in Whitehall, led by Mr Swinney for Holyrood and Treasury Chief Greg Hands for Westminster, lasted more than an hour and was focused entirely on the “central issue”; the mechanism whereby the £30 billion annual block grant to Scotland is reduced as the Scottish Parliament receives greater tax powers.

The SNP Government favours a mechanism known as "per capita indexed deduction", which would protect Scotland's budget in the likely event its population grew less quickly than England's.

Professor Anton Muscatelli, the principal of Glasgow University, has warned Scotland’s public spending would face a cut of £3.5bn or £7bn over the next 10 years if the respective alternatives of "indexed deduction" and “levels deduction” were used.

Stressing the per capita option would create "a fair deal for both Scotland and the rest of the UK," Prof Muscatelli argued this was the only system that satisfied the key “no detriment” principle set out by the Smith Commission; that is, that whatever deal is struck neither the UK nor Scotland should be worse off.

The talks’ deadlock revolves around this principle.

It is believed the UK Government's initial offer would - from Edinburgh's viewpoint - have seen Scotland losing £7bn over 10 years. It is also believed the Treasury offered a so-called compromise but this - again from Edinburgh's viewpoint - would have left Scotland £3.5bn worse off.

One SNP Government insider noted: "It's not some detriment, it's no detriment."

However, on the flipside, the UK Government believes it has moved "a long way" to address Edinburgh’s concerns.

It argues Mr Swinney's preferred per capita option would be detrimental to the UK taxpayer since, as the UK population grew faster than Scotland's, then the UK taxpayer would be expected to pay more to help fund Scotland's public services.

Given Scotland per head already receives around £1200 a year more in public spending than the rest of the UK, the Treasury believes this option is politically unsellable.

More phonecalls between officials from the two governments will take place this week with the possibility of a ninth meeting of the JEC in Edinburgh early next week.

However, with the deadlock remaining and no breakthrough looking likely, it is possible that the negotiations could go right to the wire on February 12.

Meantime, the issue is set to attract more heat this week when, on Wednesday, Mr Hands is grilled about the framework by MPs on the Commons Scottish Affairs Committee and Labour holds a Westminster debate on the subject.