Oil and gas revenues are expected to plunge from 0.4% of the UK economy to an almost negligible 0.03% by 2040, according to the UK Government's fiscal watchdog.
The campaign to keep the UK together said the Office for Budget Responsibility's (OBR) figures expose a "black hole" in the plan for Scottish independence, with oil comprising a large part of Scotland's economy.
The Scottish Government has recently begun challenging the OBR's forecasts and have proposed a "Scottish OBR" to create its own forecasts.
The latest oil and gas projections coincide with a separate report which concluded that the decline in the North Sea is already holding back the UK economy, prolonging the recession and stifling economic growth.
Better Together chairman Alistair Darling said: "It is absolute madness for the SNP to base their case for separation around a commodity that is declining and volatile.
"The SNP promise the earth on public services, welfare, pensions and an oil fund off the back of oil and gas revenues."
He added: "We know that privately the SNP accept the OBR's figures, yet in public they cook the books and predict an oil boom."
The UK OBR was created in 2010 to predict the UK's future economic performance.
It has faced criticism from the Scottish Government for allegedly over-optimistic projections of devolved tax income and allegedly pessimistic projections of future oil revenues.
Finance Secretary John Swinney recently announced that an independent financial institution for Scotland similar to the OBR is under consideration.
The plan is due to be scrutinised by Holyrood's Finance Committee, which has two SNP MSPs at its head.
Mr Darling's Labour colleague Margaret Curran, the party's shadow Scottish Secretary, said: "This OBR report confirms what the SNP say in private, but won't admit in public: that oil and gas revenues from the North Sea are volatile and seriously diminishing.
"Even John Swinney has admitted that you can't argue with the independent OBR.
"A separate Scotland would be significantly more reliant on oil and gas - comprising up to 20% of total Scottish revenues, compared to just 1-2% of UK revenues.
"Being part of the UK means we're part of a larger and more diverse economy that's able to cope with shocks and fluctuations in oil revenues."
Conservative MSP Murdo Fraser, convener of Holyrood's Energy Committee, said: "In the past the SNP has accepted the credibility of the OBR forecasts.
"Now these very figures show, even on the most careful of projections, there would be an £8.3 billion black hole in the finances of an independent Scotland, compared to what the Scottish Government promises.
"When more optimistic forecasts are considered, that gap increases to £23.9 billion."
The Centre for Public Policy for the Regions (CPPR) today produced a separate analysis of the UK's recent 0.5% annual growth, concluding that the recession would have been shorter and the recovery more pronounced if North Sea oil and gas were removed from the equation.
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