THE Treasury's top economists have undertaken the "most significant in-depth analysis" to date of the Scottish Government's economic claims, spending months number-crunching key sections of the independence White Paper.

In a multi-part fiscal strategy, which the Lib-Con coalition hopes will swing the momentum of the referendum back in its favour, the first part will come tomorrow with a keynote speech from Danny Alexander, the Chief Secretary to the Treasury.

In a "myth-busting" speech in Edinburgh, the Highland MP will seek to expose what the No camp believes are the over-optimistic claims Alex Salmond and his colleagues have made on North Sea oil revenue. His address will coincide with the publication by HMRC of annual oil and gas receipts.

A senior Whitehall source said the Treasury analysis would set out the impact of having to absorb higher spending and lower tax revenue caused by declining oil revenues, an ageing population, the SNP Government's uncosted policy pledges, and the set-up costs of independence within a much smaller budget.

"This is a significant intervention," said one senior coalition insider. "The Treasury's leading economists have produced the most significant in-depth analysis ever done in this area. We intend to up the pressure on the Scottish Government to make them come clean about their figures."

The key part of the fiscal attack will come in a few weeks' time and could coincide with another visit north of the border by Prime Minister David Cameron, who, once the May 22 European and English local elections are out of the way, will clear the decks to concentrate on saving the Union.