Last-minute talks aimed at averting a three-day strike by P&O oil tanker drivers were being held in the early hours of Friday morning. It is hoped a deal can be reached to head-off the threat of industrial action, called over a pay dispute.
Action would hit Shell petrol stations, which are supplied by P&O's Trans European branch.
Union leaders claimed Shell would start to feel the effects of the stoppages within 24 hours.
A spokeswoman for the oil giant said petrol stations in cities would be hardest-hit, although contingency plans were in place.
A spokeswoman for the Transport and General Workers' Union (TGWU) said the talks were expected to continue until around 0500 BST.
She said that unless a deal was reached the action, supported by 269 drivers in a ballot, would continue as planned.
The drivers are demanding a pay increase of 8%, and have rejected the latest offer of 4.5%.
"Shell stations only have enough supplies to last about a day," the spokeswoman added.
"They will start to feel the effects about a day after the strike begins."
A spokesman for P&O Trans European said it hoped to reach a deal with the drivers.
Shell was also hopeful that a solution could be found and that petrol stations would remain open.
But a spokeswoman warned: "There may be some inconvenience to our customers, but this will depend on individual service stations.
"Some stations, in particular the rural ones, may have enough fuel to cope with it and will last longer than those in busy city centres."
She added: "The Shell network only accounts for 10% of the country's petrol, so 90% of the country will not be affected by the strike." ©