The amount of tax expected from North Sea oil and gas has been reduced by a UK economic forecaster.
Revenue will drop from £34 billion to £33 billion between 2012-13 and 2017-18, according to the independent Office for Budget Responsibility (OBR).
The change appears to leave an £8.5 billion gap between the OBR's revised figure and the Scottish Government's least optimistic estimate.
The best-case scenario, published in an analysis paper by the Scottish Government last week, predicted oil tax revenue could be as high as £11.8 billion by 2017-18, enough to fund a third of the administration's current annual spend.
It suggests the industry could generate as much as £57 billion in tax revenue over the six years to that point.
Michael Moore, the Scottish Secretary in the UK Government, said: "There is a gulf between those independent OBR figures and the hugely optimistic numbers published by the Scottish Government last week.
"Their most optimistic scenario puts revenues in 2017-18 at three times the independent OBR figure, despite the Scottish Government's own expert group warning them to be cautious and John Swinney's own leaked Cabinet paper saying that the revenues are volatile and declining.
"The contrast here is between independent OBR figures and an exercise in wishful thinking, with no working attached, from the Scottish Government. This debate must be based on fact, not wishful thinking."
But Finance Secretary John Swinney said the OBR predictions are deficient in their assumptions of the number of barrels being extracted and in the cost per barrel.
Both factors are being underestimated when compared with other models, including the industry and the UK Government's Department for Energy and Climate Change, he said.