THE SNP's demand for devo max, or full fiscal autonomy from the rest of the UK, would lead to "austerity max," Jim Murphy has claimed.

Writing in The Herald, the Scottish Labour leader says relying on North Sea revenues at a time of low oil prices would have a "devastating" impact on public spending in Scotland.

Mr Murphy's warning comes as a report from the House of Commons Scottish Affairs Committee says full fiscal autonomy would reduce Holyrood's budget by £6.5billion immediately, a sum equivalent to more than half NHS Scotland's budget.

The Scottish Government is due to publish the annual Government Expenditure and Revenue in Scotland (GERS) report, which shows the state of the country's public finances, tomorrow (Weds).

Last year's figures, covering the financial year 2012/13, showed Scotland was deeper in the red than the UK as a whole to the tune of £283 per person.

The latest set of figures will cover 2013/14, a period ending before global oil prices began to plummet last summer.

Nicola Sturgeon has said the SNP will press for full fiscal autonomy if the Nationalists hold the balance of power at Westminster after the election.

Under the set-up, the Scottish Government would become responsible for almost all tax-raising in Scotland.

Public services would be paid for by taxes raised in Scotland, rather than a block grant from Westminster adjusted by the Barnett Formula, the mechanism for passing on a share of spending increases south of the Border.

Attacking the plan, the Scottish Labour leader writes: "We will know from the SNP's own figures tomorrow what impact scrapping Barnett and relying on oil would have on Scotland.

"If previous years are anything to go by, losing that extra share of UK tax revenue through Barnett would be devastating for our ability to pay for our schools and hospitals.

"We would also be replacing a UK-wide pension with a separate Scottish one that would be at the mercy of plummeting oil prices."

He adds: "This isn't about rerunning the arguments from the referendum debate.

"The General Election is now about the best form of devolution for Scotland within the UK."

Commenting on the Scottish Affairs committee's claim that a Holyrood government would lose £6.5billion, he says: "That's not just austerity, it's austerity max."

In its report, the committee, headed by Labour's Ian Davidson, concludes that a fiscally autonomous Scotland would receive £1.5billion in oil revenues at currency prices, while losing an £8billion fiscal transfer from the Treasury.

The report says: "The collapse in the oil price is a stark reminder of the risks that face economies which rely on a volatile revenue stream to fund a large proportion of their public spending."

Mr Murphy is due to meet oil industry leaders in London today when he will repeat calls for Chancellor George Osborne to support the industry in his Budget later this month.

Meanwhile N-56, a business group established by leading pro-independence campaigner Dan Macdonald, called for policymakers responsible for oil and gas industry taxation and regulation to be relocated from London to Aberdeen.