A forthcoming analysis paper from the Coalition has been undermined even before it has been published, the SNP has claimed, after a leaked draft showed it had planned to use the UK's coveted triple-A status as a key argument against Scottish independence.

Yet in February Moody's, one of the three main credit ratings agencies, downgraded the UK from AAA to AA1 and last month Fitch followed suit.

Standard and Poor's, the third leading agency, has maintained the UK's AAA status.

The third UK Government analysis paper is due to be published in the next few days and will focus on financial services. It is expected to claim that because borrowing costs would be higher in an independent Scotland – due to it having no individual track record – then insurance and mortgage costs to consumers would rise too.

The paper is also likely to suggest there would be an "independence flight" with finance companies relocating south of the Border.

Whitehall has already highlighted how life insurers based in Scotland sell only 6% of their products to Scottish postcodes and 94% to the rest of the UK while mortgage providers based in Scotland sell just 16% of their mortgages in Scotland and 84% elsewhere in the UK.

The SNP said it had obtained a Treasury paper entitled Impacts on Mortgage Market.

The paper states: "The expectation is that an independent Scotland would not have a AAA credit rating it currently enjoys."

This, it argues, would lead to dearer mortgages under independence.

The SNP said despite the triple-A downgrade, the No campaign continued to use it in its documents.

"This leaked working paper exposes the utter weakness of the Treasury's case," declared Stewart Hosie, the Nationalists' Treasury spokesman.