TAX revenues from the North Sea have fallen to their lowest level since the 'black gold' was first pumped ashore to Scotland four decades ago.

In what was described as a "historic moment" for the UK, which has reaped billions of pounds since North Sea production began in 1975, taxes plummeted to a meagre £35million during the financial year just ended.

The slump from a flood of cash just a few years ago to a trickle follows falling production, higher operating costs and the collapse of global oil prices.

The industry, which has already contracted sharply, admitted it would struggle to sustain even its current scale with oil hovering around $40 dollars a barrel.

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But Mike Tholen, economics director of Oil & Gas UK, insisted that while tax revenues would fall, the industry would "remain a significant employer".

The figures also prompted questions of the SNP, which predicted oil revenues of nearly £7billion this year in its blueprint for independence.

Willie Rennie, the Scottish Liberal Democrat leader claimed the Nationalists' credibility "has been damaged irreversibly".

The latest figures, from HM Revenue and Customs, showed the Treasury received £538million in offshore corporation tax but paid out rebates of £503million in respect of the other main North Sea levy, petroleum revenue tax, to loss-making producers hit by the downturn in the industry.

The £35million total is the lowest since 1975/76, when receipts were £25million.

To put it in context, the Scottish Government spends £35million every eight hours approximately.

It was down from £2.1billion the previous year and almost £11billion as recently as 2011/12.

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The picture is set to deteriorate further, according to the most recent forecasts.

In its analysis accompanying the Budget last month, the independent Office for Budget Responsibility said revenues would be negative up to 2020/21, the end of its forecast period.

The Treasury, it predicted, would pay out £800million per year, on average, over the period, as rebates exceeded receipts.

Mr Tholen said: "At around $40 per barrel, oil is still more than 60 per cent lower than it has been over the last three years.

"In these conditions, the UK North Sea industry will continue to struggle to sustain its current scale.

"Despite the projected fall in production taxes which is a consequence of the current low oil prices, industry will remain a significant employer, provider of energy security, hub of innovation and leader in the export of good and services to overseas markets."

He said the industry had generated more than £330billion in tax revenues at today's prices since production began.

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Economist John McLaren said: "This is a historic moment.

"The future of the North Sea is now about saving jobs and keeping the industry alive. It is not about tax revenues.

"To that extent, North Sea revenues are no longer part of the political debate in Scotland."

The first oil was pumped ashore to Scotland from the Argyll field in 1975 following a string of exciting discoveries in the late 1960s and early 1970s.

In real terms, revenues peaked in the early 1980s, when the Treasury collected up to £30billion a year at today's prices.

In the Budget, George Osborne took action to shore up the struggling oil industry, which has already shed 65,000 jobs, offering tax breaks worth £1billion.

The Scottish Government has called for further action to encourage the decommissioning of facilities.

It has set up a fund to support oil workers who lose their jobs.

The figures came a day after Nicola Sturgeon announced a fresh campaign to win support for independence.

The SNP is in the process of developing a new economic case for leaving the UK, following the collapse of the oil price and failure of its currency policy during the 2014 referendum.

Mr Rennie said: "The SNP leadership's judgement on the oil industry in the run up to the independence referendum was wildly misplaced and as a result their credibility has been damaged irreversibly."

An SNP spokesman said: "Scotland remains the biggest oil producer in the EU and there are estimated to be more than 20 billion barrels remaining in the North Sea but onshore revenues in the next few years are predicted to more than offset any decline in offshore income.

"The SNP Government's oil price forecast was in line with most international estimates and below the UK Government's own prediction.

"But the case for an independent Scotland was not, and is not, founded on the price of oil."