FUTURE oil revenue forecasts produced by the Scottish Government in the run-up to the independence referendum have been proven wildly optimistic, after it emerged that tax receipts were more than £1 billion lower than even the worst-case scenario presented to voters.

In May last year, a publication that was vital to building the SNP's case for independence set out five possible scenarios for oil and gas receipts in coming years, claiming revenues could be as high as £5.8 billion in 2014-15, with £3.3 billion the least optimistic.

However, in the period, Scotland's share of tax receipts generated by North Sea oil was just £1.9 billion, HMRC figures have confirmed.

The publication of the statistics follows the SNP's refusal to release new North Sea revenue projections despite the party's bid to secure devo-max for Scotland, a system under which oil taxes would prove vital in funding public services.

The forecasts presented in the run-up to the referendum and ministers' subsequent refusal to release updated forecasts was branded "nothing short of a political scandal" by Alex Johnstone, Tory MSP for the North East.

He added: "As the SNP sought to persuade people of the financial case for independence, it tried to dupe the Scottish people by using numbers it knew to be utterly false.

"Fortunately, Scots decided against independence last year. But the SNP is still pushing ahead with a plan for a second referendum and it still wants to tear up Britain's tax system. That means we need to have straight facts and answers from the SNP on oil."

Nicola Sturgeon has been under pressure since January to order the publication of updated forecasts, and last month promised a new bulletin "as soon as possible". However, this week her spokesman said that would now be impossible ahead of the general election, as the pre-election period meant civil servants cannot make politically sensitive announcements.

The SNP was accused of picking and choosing when to follow the rules, after Ms Sturgeon announced almost £1m of additional funding for Trade Unions this week, a move likely to appeal to traditional Labour voters.

The HMRC figures show that in 2014-15, the Treasury received just £75 million in Petroleum Revenue Tax, the second lowest total since the charge began. Offshore corporation tax made just over £2 billion. A fiscally autonomous Scotland would be entitled to 90 per cent of the revenue.

Ahead of the referendum, the Government had forecast an oil price of around $110 a barrel in the years ahead, with Alex Salmond predicting a "second oil boom". However, the price collapsed to less than $50 towards the end of last year, before rising slightly to around $60.

In the White Paper, the SNP put forward two projections for oil tax revenues in 2016-17, £6.8 billion and £7.9 billion.

Economist John McLaren, of the independent think tank Fiscal Affairs Scotland, said the Scottish Government had presented "a variety of upside scenarios but no downside" when coming up with its predictions.

He added that it was "unclear" why officials had been unable to produce an updated oil and gas bulletin to reflect to declining oil price, saying they could have chosen to do so at any point.

He added: "It isn't true to say no-one saw a fall in the price coming, Both Wood Mackenzie and the International Energy Agency (IEA) had been predicting a potential fall for some time."

A spokesman for energy minister Fergus Ewing said oil was a "bonus, not the basis" of Scotland's economy. He added: "Most independent forecasts expect the price to rise again, with OPEC predicting a price of $110 per barrel for the rest of the decade and around $100 in real terms in the long-run.

"Not a single penny of Scotland's oil wealth over the last 40 years has been invested for the future, due to the failure of successive Tory, Labour and Lib Dem Westminster governments."