Scotland's First Minister is being unfair to voters in the independence referendum by "promising milk and honey" when he should be spelling out the "painful choices a separate Scotland would have to make", a leading UK Government minister has claimed.

Danny Alexander, chief secretary to the Treasury, said with voting in the crucial ballot now only weeks away that Alex Salmond is "still making plans based on untold oil wealth beyond anything independent forecasters consider plausible".

He was speaking after the UK Office for Budget Responsibility (OBR) revised down the amount of cash it expects to be raised from North Sea oil and gas revenues.

Chairman Robert Chote revealed the independent body is now forecasting revenues of £61.6 billion will be raised between 2013/14 and 2040/41 - down from £82.2 billion.

The First Minister has already dismissed the OBR's figures as "stuff and nonsense".

But in a letter to the SNP leader, Mr Alexander hit out: "This stage in the referendum campaign should be about presenting the people of Scotland with the facts about what separation would mean. Instead you persist in offering a fantastical picture of a separate Scotland's public finances."

The Scottish Government argues the OBR forecasts are based on a "very low estimate of future total production".

It has already published higher forecasts for oil and gas income, saying this could be between £2.9 billion and £7.8bn in 2016/17 - the first year of an independent Scotland.

It further predicts revenues could then amount to £34.3bn over the next five years - equal to almost £7bn a year.

But Mr Alexander insisted the revisions by the OBR made Scottish government forecasts "look even more incredible".

He said: "It is clear to every independent expert that there is no boom in oil revenues on the way, and because extraction is expected to become more expensive, we have to be realistic about tax revenue projections."

A spokesman for Holyrood energy minister Fergus Ewing hit out: "Danny Alexander must apologise for the Treasury's 'dodgy dossier' on the finances of an independent Scotland, just as he should apologise for his leading role in Westminster's 2011 tax grab on the North Sea, which he boasted about before being forced to scrap it.

"Mr Alexander's oil figures have been described as missing 'a mountain of black gold' by respected economist Professor Sir Donald MacKay. North Sea oil is a huge asset and will be for many decades to come and the OBR's forecasts rest on estimates of future production which are well below those used by the industry, by leading experts and by the UK Government."