Hundreds of drivers who supply fuel to almost 1,000 Shell garages as well as factories and some smaller airports will walk out from next Friday unless the dispute is resolved.

The drivers, most of whom are employed by Hoyer on Shell contracts, voted last week in favour of industrial action. The union is campaigning for a minimum wage of £36,000. The current level is £34,000.

The union said Shell was making huge profits but accused the company of forcing through cost-cutting in its supply chain.

The two sides have agreed to go to the conciliation service Acas in the next few days in a bid to avert industrial action.

Hoyer said it had made an improved pay offer worth 6.5% on wage rates plus an extra 0.3% on other elements, making the deal worth 6.8%.

The offer would raise drivers' average earnings from £36,000 to around £39,000, said the company.

Bernie Holloway from Hoyer said: "It is a great disappointment that the trade union have decided to reject the improved offer.

"We believe this is a very good offer, and combines with previous pay awards to produce an overall increase in average earnings over the last four years totalling 27% - double the rate of inflation. However, the union have responded by re-enforcing their demand for a 13% increase."

Unite national officer Ron Webb said: "It is time that Shell intervened directly and stopped leaving this problem to the contractor. Shell makes profits of £1 billion a month so there is no excuse for allowing these drivers to be paid no more than they were 15 years ago."