An independent Scotland would have a lower credit rating than the UK and be forced to pay more to borrow money, a leading ratings agency has warned.

Moody's also backed the UK Government's refusal to agree to a currency union with a separate Scotland.

Better Together campaign leader Alistair Darling said the findings made "clear that if we leave the UK then we would pay more for our mortgages, credit cards and loans". But the Scottish Government rejected the analysis, saying Scotland was in a stronger position economically than the UK. Moody's analysis said an independent Scotland's credit rating would be at least two notches below that of the UK. An "A" rating was "perhaps the most likely at the outset", it said, "but with risks to the downside.

Such a rating, around the same as that of Botswana, could lead to higher borrowing rates on the international markets.

As part of three separate reports on Scotland, Moody's also warned that independence could cut returns on energy investment and affect Scotland's insurance industry. But it was "unlikely" that independence "would have a materially detrimental effect on the medium-term performance of the Scottish economy", it added. Moody's also suggested independence could improve the UK's credit rating. Agreeing to a currency union with an independent Scotland, however, would be "credit negative".

The reports suggest Alex Salmond's threat not to accept any national debt unless Scotland is allowed to share the pound would also be a "risk" to the UK's credit worthiness.

Mr Darling described the reports as "absolutely devastating" report for the nationalists.

Danny Alexander, the Chief Secretary to the Treasury, said: "Moody's confirm why the UK should not join a currency union - because it would then also have a weaker credit rating. Together as the United Kingdom we are all better off."

A spokesman for Scottish Finance Secretary John Swinney said: "An independent Scotland will be the 14th wealthiest country per head in the developed world, compared to the UK in 18th.

"Year-on-year, Scotland is in a stronger financial position than the UK as a whole. And Scotland's public sector debt is lower than the UK's, and lower than the EU and G7 average, confirming our underlying economic strength."