AN independent Scotland would most likely not enjoy the AAA credit rating currently assigned to the UK as a whole, a leading fund manager has warned.

Jim Leaviss, head of retail fixed interest at M&G Investments, the fund management arm of insurer Prudential, said ratings agencies would be concerned about Scotland's size and its ability to service its debt.

"My guess is that a rating agency would not give Scotland a AAA rating, and that the market would trade any bonds it issued at a wider yield than the UK," Mr Leaviss wrote in a note.

This would mean an independent Scotland facing higher borrowing costs than the UK Government.

On independence Scotland would likely become liable for 8.4% of the national debt, some £80 billion.

Mr Leaviss noted that "an independent Scotland looks AAA on the back of an envelope (as long as it gets all of the oil and none of the banks)". This is because to be AAA-rated, a country should usually have a debt to gross domestic product ratio of 60% or below and interest costs of less than 12% of revenues. Scotland is likely to come in at 56% and 8% respectively, he calculated. But Mr Leaviss, a former Gilts trader for the Bank of England, noted there is uncertainty about responsibility for additional debts from the rescue of banks such as RBS.

He said there would be concern on a budget deficit he estimated at 9% and Scotland's relatively lacklustre average growth rate.

The UK's ability to print its own currency underpins its AAA rating, he argued. But an independent Scotland would most likely use the euro or UK pound.

"Additionally, ratings agencies favour big, systemic nations over smaller ones where there is an implied higher risk factor (rightly or wrongly) and given the experience of Iceland and Ireland, which both held AAA ratings, perhaps the agencies would err on the side of a lower rating," he added.

Mr Leaviss's research comes at a time when UK gilt yields are at near record lows as investors seek shelter from turmoil in euro debt markets.

His intervention in the independence debate follows that of Citigroup analyst Peter Artherton who argued in November that the SNP administration's plans to boost renewable energy generation conflicted with its independence aspirations.

The SNP is committed to holding a referendum on independence before 2016.