ALEX Salmond is facing growing pressure to update his oil revenue forecasts after estimates used by the UK Government were cut by nearly £3billion.

Politicians and economists called on the Scottish Government to revisit a key report from last year which predicted the country would raise at least £34.6billion in North Sea taxes between now and 2017/18.

The demands came after the independent Office for Budget Responsibility again downgraded its forecasts in figures published alongside the Budget.

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According to the OBR, the UK will collect £18.8billion from oil and gas tax over the same period.

The more optimistic Scottish Government estimates - issued a year ago - have been used repeatedly by the Nationalists as the basis of claims that Scotland would be more prosperous as an independent country than if it remained part of the UK.

Scottish Conservative leader Ruth Davidson said: "Alex Salmond has consistently made wildly overstated claims about the revenue collected from oil to help fund his separation dream.

"He needs to bring forward revised oil figures now, so people in Scotland have a better idea of the real choice facing our country in September."

The Scottish Government's most cautious estimate for oil revenues in 2016/17 - the year it plans to declare independence - is £6.8billion.

The figure is more than double the OBR projection of £3.2billion for the same year.

The Scottish Government report making the predictions, the Oil and Gas Analytical Bulletin, already appears to have overestimated revenues for the current financial year.

It predicted receipts of up to £8.3billion for 2013-14 but with 10 months' tax so far collected, the total has only reached £4.7billion.

The call for updated figures was backed by economist John McLaren of the CPPR think-tank based at Glasgow University.

He said: "The Scottish Government's figures are more than a year old and since then the estimates on which they are based have been updated twice by Oil and Gas UK and twice by the OBR and Department of Energy and Climate Change.

"An update is well overdue."

Mr Salmond was pressed on the issue during First Minister's Questions at the Scottish Parliament when Ms Davidson's hit out what she called his "dodgy" forecasts.

Mr Salmond refused to say whether updated figures would be produced but said: "The figures we have outlined are robust."

He said the Scottish Government's figures were based on a lower estimate of oil prices than used either by the OBR or DECC.

He added: "Given the evidence of the last 40 years, I think people are more likely to believe a Scottish Government forecast and estimations of Scottish natural resources than a Tory Party who have pocketed £300billion of Scottish resources over the last 40 years."

Last year's Scottish Government oil analysis report was announced as the first in a series.

A second study was published in November but did not update the forecasts, which were quoted in the SNP's independence White Paper.

The First Minister's chief political spokesman confirmed there was no date for the next analysis paper. He was unable to say whether it would update the forecasts.

On Wednesday, the OBR downgraded forecasts by £2.9billion, between now and 2018/19, compared with its previous estimates issued in December.

In his Budget speech, Chancellor George Osborne claimed the figures showed the "precarious" nature of an independent Scotland's finances.