A pipeline carrying up to half of Britain's oil was gradually being shut down on Saturday ahead of a strike over pensions that has already closed a major refinery and prompted some panic fuel buying.
The Grangemouth refinery in Scotland produces a tenth of Britain's petrol and diesel but also has a power station that supplies the neighbouring Kinneil plant that processes the crude oil coming ashore from 70 fields in the North Sea.
Without that power Kinneil cannot operate. In preparation, several of the North Sea fields have begun to cease production.
"We have been told we will receive power until the strike starts, so we will begin ramping down the pipeline system overnight and by 6am it will be completely shut down," a spokeswoman for operator BP said.
The strike is the first to close a British refinery in over 70 years.
The Forties pipeline carries an average of 700,000 barrels per day (bpd), close to half the 1.5 million barrels the country produces daily. One fifth of Britain's gas supply also relies on the Forties system.
The Forties oil alone is worth some Stg£50 million a day and its closure for the two-day strike will make a significant dent in already stretched government coffers because of hefty tax revenues.
Management at the 200,000 bpd Grangemouth refinery, owned by international chemical company Ineos, met officials of the UNITE union today but failed to get a deal that would allow the pipeline to continue operating.
"We will provide full safety cover over the two days so the refinery can be fired up quickly after the strike but the Kinneil plant will not be treated any differently from any other facility on the site," a union spokesman said after the meeting.
He said no further meetings were planned with management.
The government has called on Ineos and the unions to cooperate to ensure the power station remains open.
It insists there will be no overall shortages of fuel but accepts that there may be some local supply problems, particularly in Scotland and northern England.
"There is plenty of petrol and diesel in Scotland to meet demand during this period of time. But of course that is going to be challenged if people change the way that they consume fuel," industry minister John Hutton told BBC radio.
BP said that assuming it got power back as soon as the strike ended and Forties fields resumed production rapidly, the pipeline could be back in operation within 24 hours but might take a few more days to get back to full flow.
Ineos said yesterday that it had completed the shutdown at Grangemouth. Talks to resolve the dispute collapsed on Wednesday evening.
European diesel prices have shot up this week on worries about a fuel shortage.
British gas prices have also jumped although the National Grid gas supply network said it did not expect gas shortages as warm spring weather has curbed demand.