THE latest Treasury assault against independence appeared to have backfired last night, as the SNP seized on a Whitehall confirmation that Scotland has the strongest economy in the UK outside London and the south-east, which, said Finance Secretary John Swinney, proved that Scotland could be an independent success.

The Treasury is due to issue an analysis of Scotland's tax-and-spending position next month as part of its case for retaining the UK.

It will argue that Scotland's economy benefits immensely from the Union, and that a Yes vote in September would put that benefit at risk.

It is expected to focus on the steep fall in taxes from Scotland's North Sea oil and gas - down 42% in 2012/13, according to Citigroup analysts last week - and contrast it with rising public spending, driven in part by Scotland's growing elderly population.

The widening gap between income and demand would be a major challenge, the paper will conclude.

However, in order to support its case that Scots have much to lose, the Treasury paper contains a raft of data that also draws attention to Scotland's economic strengths.

These include employment being higher and unemployment lower in Scotland than in the rest of the UK; economic activity per head in Scotland being second only to London and the south-east; and growth in economic activity per person in Scotland outstripping or matching the UK in each of the last five decades.

The same figures are now being used by the Yes camp to claim the Scottish economy is more than capable of adjusting to independence.

By coincidence, the cross-party Yes Scotland movement tomorrow launches a £2 million advertising blitz based on Scotland's "wealth", saying Scotland can, should and must be independent.

One poster asks: "What would you say to living in one of the world's wealthiest nations?"

However, the Yes camp's focus on wealth may also backfire this week, when the Scottish Government publishes the figures for the 2012/13 Government and Expenditure and Revenue in Scotland (Gers).

These are expected to show Scotland's deficit almost matching the UK's because of a slump in oil revenue, puncturing the SNP claim that Scotland is relatively better off.

Indeed, Citigroup estimates Scotland's deficit, even taking into account a geographical share of North Sea oil, now exceeds the UK's at 8.3% of GDP, compared to a UK deficit of 6.8% of GDP.

Scotland's deficit is expected to remain worse than the UK's as North Sea oil declines.

Danny Alexander, LibDem Chief Secretary to the Treasury, said that after ruling out a formal currency union in the event of independence, the government's intention was now to inform voters about the tax-and-spending position.

He said April's fiscal analysis would look at "the various black holes circulating round the SNP's arithmetic" in both the short- and long-term.

He told the Sunday Herald: "Having settled the position on the currency, it is really important that we establish in people's minds what the reality is of the tax-and-spending position.

"As I look at the prospects for independence and some of the math that's been set out by the SNP, the challenges that would face whoever was in my job in an independent Scotland would make the challenges I've faced over the last three-and-a-half years pale into insignificance.

"On the one hand, the declining oil revenues, on the other, demographic pressures building up. The gap just gets wider over time."

He went on: "Scotland is, after London and the south-east, the most successful part of the UK economy.

"If you compare Scotland to some of the European countries who the SNP like to compare us to - Finland, Denmark, Portugal - we Scots are at least as, if not more, successful than those places, and we get that success because we're part of the UK.

"And if you pull out of the UK you're destroying one of the foundations of that success."

The Highlands MP also restated the opposition of the three pro-Union parties in relation to Alex Salmond's proposal for a sterling currency union, a position that Scotland's First Minister claims is a bluff.

Alexander said: "The idea that you go out and make a big statement [refusal] like this with the intention of changing your mind in a few months' time ... it's just completely potty."

Asked if, as a Scottish MP, he would argue for a currency union in the event of independence, he said: "No, I absolutely wouldn't.

"A currency union would not be in Scotland's interests."

He said that trying to establish a new state "with both hands tied behind your back" - no ability to set interest or exchange rates - was "economic nonsense".

Scotland's Finance Secretary, John Swinney, said: "We welcome the fact that even the Treasury clearly agree that Scotland has an immensely strong economy and can be a successful independent country.

"The Chief Secretary of the Treasury must now accept that Scotland has what it takes to be a successful, independent country.

"In each of the last 30 years, Scotland has generated more tax receipts per person ... and our long-term finances are in a stronger position than the UK with a relative surplus."