NorthLink won the contract in October 2002

The figure, released by Audit Scotland, is more than double the subsidy agreed when NorthLink ferries took over the service in 2002.

Audit Scotland said the Scottish Executive had little option but to begin to re-tender the routes early.

The SNP said tendering the routes had been a waste of time and money.

Financial difficulties

The financial watchdog was called in to look at the executive's handling of the Northern Isles ferry contract after concerns over how much money NorthLink was getting to run it.

In its report, the watchdog said the ferry operator got into financial difficulties because competition on the routes reduced its income, and there were unforeseen operating costs such as the price of fuel.

The Auditor General for Scotland Robert Black found the executive's transport department conducted an "adequate" tendering exercise, having awarded NorthLink a five-year deal as it submitted the lowest bid for subsidy.

However, Mr Black said the probe by Audit Scotland highlighted the need for would-be operators to be given proper information and for the executive to fully examine bidders' cost estimates.

Transport Minister Tavish Scott has pledged to follow the auditor's recommendations.

'Beggars belief'

Before it took over service in 2002, NorthLink had expected a �29.7m basic subsidy over the first three years plus inflation.

Opposition parties claimed taxpayers were picking up the tab for the executive's mistakes.

SNP deputy transport spokesman Brian Adam said: "It beggars belief that this futile exercise in tendering was in the public interest."

Highlands and Islands Tory MSP Jamie McGrigor said: "In the end, the taxpayer has been forced to pick up the bill because the executive got it so wrong right at the beginning."