THE SNP has come under renewed pressure over its plan to demand devo max after oil revenue forecasts were downgraded by billions of pounds.

The independent Office for Budget Responsibility slashed its forecasts for North Sea taxes by £9.6billion over the next five years.

The North Sea - which has seen oil prices halve since last summer - is expected to generate just £3.5billion between next financial year and 2019/20, equivalent to 15 weeks' funding for NHS Scotland.

The £600million forecast for 2016/17 was more than ten times smaller than the £6.8billion estimate used by the Scottish Government in its independence White Paper last year.

Labour and Conservative MSPs warned the slump would wreck Scotland's public finances under devo max, a system that would make Holyrood reliant on raising all its own taxes.

Scottish Labour's finance spokeswoman Jackie Baillie said: "Today's figures expose the devastating consequences of the SNP's General Election plan for full fiscal autonomy.

"Scrapping the security of the higher public spending we get through Barnett for over reliance on volatile oil prices would mean only one thing - austerity max from the SNP.

"It's clear that only Labour can credibly claim to be the anti-austerity party of Scotland.

Scottish Conservative energy spokesman Murdo Fraser said:

"Had Scotland voted Yes, we'd be staring down the barrel of a gun.

"Indeed, even with the SNP's full fiscal autonomy proposals, Scotland would be left having to slash public services, hike up tax significantly, or vastly increase borrowing.

"Thankfully, with the strength of the UK to endure this, the disappointing years can be ridden through."

The OBR figures were slashed compared with its last set of predictions in December partly as a result of new North Sea tax breaks - backed by all the parties - announced by the Chancellor in his Budget.

Oil taxes are expected to bring in £2.6billion this year, £200million less than originally forecast.

In 2015/16 the total will fall to £700million, it's lowest level as a proportion of national income since 1975.

It will remain under £1billion for each of the following four years.

First Minister Nicola Sturgeon has said the SNP will press for devo max, or full fiscal autonomy, if it emerges from the election in a position to influence the next UK government.

She restated the demand last week, as she launched official figures showing Scotland was deeper in deficit than the rest of the UK.

A Scottish Government spokesman said its White Paper oil forecasts were "in line" with other predictions at the time.

He added: "It is clear that the industry is currently facing a range of challenges including lower oil prices and high costs and the UK Government's record on fiscal instability is partly to blame for the decline in oil and gas revenue.

"We welcome the Chancellor's U-turn to correct his previous fiscal mismanagement - however, this will have an inevitable impact on tax revenue in the short term. "These measures will help to ensure that North Sea activity rebounds in the medium and longer term."