Alex Salmond's economic case for independence endured a blow after it emerged that oil revenues are due to decline sharply for two successive years, deepening Scotland's spending deficit.

Scotland's annual accounts disclosed that the country had a deficit of £12bn last year, after the country's share of North Sea oil revenues fell by 41.5% in 2012-13, cutting those revenues by £4.5bn from the previous year.

The Treasury then disclosed that this financial year's oil tax receipts were currently 24% lower than last year's. That puts Scotland's geographical share on course to fall from £5.6bn in 2012-13 to roughly £4.2bn this year, implying an even deeper overall public spending deficit.

The figures, from the annual Government Expenditure and Revenues Scotland report, have come at a crucial time for the Scottish government, with six months to go before the independence referendum.

Alistair Darling, the Labour former chancellor who chairs the pro-UK Better Together campaign, said the figures raised critical questions about the Scottish government's heavy reliance on oil receipts to fund high levels of public spending.

"If Scotland was independent today we would have no option but to cut spending on services like schools and hospitals or put up taxes – or probably both. Today as part of the UK we don't have to do that," Darling said. "The drop in oil revenue is so big that it is the equivalent of the entire budget for Scottish schools."

The first minister fended off the attacks, insisting that last year's decline was a short-term bump. Thanks to several boom years with oil revenues, Scotland's economy had still been £8.3bn better off overall over the last five years than the UK's, he said.

Speaking before the Treasury released the latest HMRC tax figures, Salmond said even last year's deficit left Scotland one of the world's wealthiest nations.

He added that, despite the fact that oil revenues fell by 40%, Scotland's net deficit was only 1% higher than the UK's, showing Scotland's overall finances were in a healthy state.

"The figures show that tax revenues generated in 2012-13 were £800 higher per head in Scotland compared with the UK, meaning that now for every one of the last 33 years, tax receipts have been higher in Scotland than the UK," Salmond said.

He added that last year's fall in North Sea revenues was partly because oil firms had won tax relief because they had invested billions in new equipment, which would push up future oil production, as well as an unforeseen gas supply stoppage.

But the latest HMRC figures showed that from April 2013 to January 2014, North Sea industries had contributed £3.7bn in corporation tax and petroleum revenue tax, a fall of 24% from the same period last year when that figure stood at £4.8bn.

Tom Greatrex, the shadow energy minister, said: "These figures make clear that the decline in North Sea revenues isn't the blip that Alex Salmond wants us all to believe it is, but is in fact part of a long-term trend."

However, the Institute for Fiscal Studies said the data for 2012-13 confirmed its view that Scotland faced even greater spending cuts than the rest of Britain if oil revenues continued to fall.

"The latest figures therefore illustrate how sensitive an independent Scotland's public finances would be to volatility in North Sea revenues," the IFS said.

The Treasury said data showed public spending in Scotland increased by 0.5% in 2012-13, even while Scotland's overall tax income including North Sea oil fell by 5.6%.

That allowed Scotland to maintain higher public spending at £1,325 more per person than the UK average, since Scotland received 9.3% of UK spending from the Treasury but contributed 9.1% of UK taxes. In 2011-12 – a boom year often cited by Salmond and the Yes Scotland independence campaign in their campaign leaflets and newspapers – Scotland contributed 9.9% of overall UK taxes but received 9.6% of spending.

A Scottish government spokesman insisted they believed oil revenues would rise again, with a large majority of firms expecting production improvements this year and new oil fields due to come on stream.

"We expect that record North Sea investment will lead to an increase in revenues in coming years. Oil and Gas UK's central forecast is for production to rise from around 1.4m barrels a day in 2013 to 1.7m by 2017," he said.

In a further setback for Salmond, the financier George Soros said an independent Scotland would be far better off joining the euro than either trying to form a sterling zone with the UK or having an independent currency.

Famous for "breaking" the Bank of England on Black Wednesday in 1992 by betting against the pound, Soros said a currency union would make for "a very difficult relationship" between Edinburgh and London, while an independent currency would be "very inefficient and potentially dangerous".

"Markets, currencies, can be attacked and you can speculate against currencies, and when you have a weak currency, positions can be taken," he said in London.