The Forties pipeline system, which pumps crude oil from the North Sea, is set to shut down tonight, as 1,200 workers at the Grangemouth refinery in Scotland prepare to walk out in a dispute over pensions.

A spokesman for BP, which operates the pipeline, said that it expected the pipeline to close before power from Grangemouth was switched off late on Saturday, ahead of the 48-hour stoppage on Sunday and Monday.

Up to 50 North Sea oilfields may have to cease production when the main Forties system closes down.

The pipeline supplies 700,000 barrels of oil a day, equivalent to 20 per cent of North Sea oil production.

The shutdown will cost the UK's economy about £50 million a day, including about £25 million a day in revenues to the Exchequer.

Fears that the strike at one of the country’s biggest oil refineries could hit petrol supplies increased today when a Government minister said he could not guarantee that motorists would be able to fill up at garages.

Malcolm Wicks, the Energy Minister, said today that the pipeline was a “crucial” piece of infrastructure.

Mr Wicks, speaking on BBC Radio 4’s Today programme, said that petrol supplies across the UK should not be a problem, but he acknowledged that some motorists could be hit.

He said he could not guarantee that every garage and every forecourt would have petrol every time motorists arrived to fill up.

“I cannot guarantee that every garage forecourt will have petrol at that precise moment," he said.

"I hope the vast majority of people are sensible about this. They might have to be patient. People will have to be sensible and rational."

Oil prices have fallen this morning despite continuing supply concerns in the face of the planned strike at the 200,000 barrel per day refinery.

London Brent crude for June delivery was down $1.71 at $112.63.

Ed Meir, an MF Global analyst, said that the strike was potentially very serious for the industry. “We believe that there will be tremendous pressure on the two sides to settle," he said.

John Hutton, secretary of state for business, told MPs yesterday that fuel stocks and imports should be sufficient to maintain supplies during the strike.

Steam and electricity from the Grangemouth refinery are essential to operations at the nearby Kinneil processing plant, where crude oil from the Forties pipeline is stabilised by removing sulphur and extracting gas.

Unless Ineos can supply basic utilities to Kinneil to keep it running, oil and gas production from the Forties sector of the North Sea is likely to halt within the next 24 hours.

Tony Woodley, the general secretary of the Unite union that represents Grangemouth workers, has indicated that the strike could escalate. Mr Woodley will address a mass meeting of workers at Grangemouth today.

Mr Woodley has said that after the two-day strike there will be a pause, but he said that if the company remained intransigent then an escalation of the dispute was inevitable.

Unite has indicated that it will begin a work-to-rule after the dispute, which would make the 24 hour a day, seven day a week operation run by Ineos, Grangemouth's owner, very difficult.

“We understand the seriousness of the situation. It is extremely serious - that is why Unite has been behaving responsibly. We have made sure the plant and equipment is in a state to start up extremely quickly and we have made sure there is emergency cover for the emergency services,” Mr Woodley said.

He has accused Ineos of “going through the motions” during the two days of peace talks this week at Acas.