THE SNP's plan for full fiscal autonomy has been dealt a blow after the Scottish Government was forced to slash its own oil revenue forecasts.

The latest official estimates were billions of pounds down on figures produced in May last year that were used to argue the case for independence.

The Scottish Government predicted total revenues of between £2.4billion and £10.8billion in the four years to 2019/20.

Last year's figures - forecasting a five year period up to 2018/19 - ranged between £15.8billion and £38.7billion.

The government's most optimistic figure was based on oil prices climbing back to $100 dollars a barrel immediately, a scenario dismissed by experts.

The North Sea industry itself is braced for prices to remain around their present level of about $60 per barrel for the long term.

The figures were included in the Scottish Government's latest oil and gas bulletin, which ministers have been under pressure to update since prices collapsed in the second half of last year.

Opposition MSPs said the figures blew a hole in the SNP's plan for full fiscal autonomy, under which Holyrood would take control of all tax - including North Sea revenues - and spending decisions in Scotland.

They also voiced anger that the long-awaited report was published on the final day of business at Holyrood before parliament's long summer break.

Liam McArthur, the Lib Dems' energy spokesman, said: "The SNP Government's credibility on oil and gas is falling faster than their own massive downgrades to forecast receipts.

"The downwards revision to their most optimistic forecast would have seen a £30million cut to public services in the blink of an eye.

"This blows a massive hole in their proposals for Scotland to scrap the Barnett formula and adopt full fiscal autonomy."

He added: "Issuing this report on the day parliament rises for the summer recess either shows contempt for parliament or contempt for the oil and gas industry. "The strong suspicion is that it is both. Shame on them."

Secretary of State for Scotland David Mundell said: "By publishing on the last day the parliament is sitting the suspicion is they want to avoid scrutiny by the parliament."

He said the UK Government would reject the SNP's plan for full fiscal autonomy.

On Monday, Nationalist MSPs will seek to amend the Scotland Bill, now going through Westminster, to secure full tax powers.

But Mr Mundell, speaking after giving evidence to Holyrood's devolution committee, said: "The amendment we are not accepting is full fiscal autonomy. It would be bad for Scotland.

"It would lead to a £10billion reduction in spending by 2020. That will simply not be accepted."

Scottish Labour finance spokeswoman Jackie Baillie said: "These new figures blow the SNP's policy of full fiscal autonomy out of the water.

"Last week the SNP trooped through the lobbies with the extreme right wing of the Tory party to vote for full fiscal autonomy. "It is as clear as day now that they knew the policy would be a disaster for Scotland - the SNP Government's own figures prove it."

The SNP's case for independence was based on an oil price of $110 per barrel, nearly double today's level.

The government's latest bulletin said that prices remained "substantially lower" then predicted by forecasters 12 months ago.

Ministers attempted to put a positive gloss on the state of the North Sea industry.

Deputy First Minster and Finance Secretary John Swinney said there was "no disputing" that the industry has faced "a very challenging year".

But he added: "These figures show that considerable opportunities to extend production remain in the UK continental shelf and that, properly supported, the industry can boost production over the next five years."

A spokesman for First Minister Nicola Sturgeon dismissed claim about the bulletin's timing.

He said: "It was published today because it was ready to be published today, simple as that."