Lead replacement petrol, or LRP, is likely to disappear from many petrol stations in the autumn, leaving an estimated 1.5 million drivers in the lurch.
The AA yesterday expressed concern, urging that forecourts display warning of the change, together with advice on how to modify unleaded petrol. John Stubbs, the AA's head of technical policy, said: "We feel retailers owe it their customers to give fair warning and advice when dropping LRP."
Leaded petrol was banned in 1999 due to concern over toxic emissions. Since 1992, all new cars must have a catalytic convertor to take unleaded fuel. Many older vehicles cannot take unleaded - pre-1983 Ford Fiestas and pre-1986 Escorts, most Sierras, 1980s Minis, Metros, Maestros and Montegos, pre-1985 Renault 5s, and Austin Allegros.
The department of transport yesterday issued guidelines advising drivers to buy lead replacement additives to be added to petrol using syringes.
LRP is 2% of petrol sales. A BP spokesman said it did not yet have a withdrawal date, but said: "There will come a point where the percentage becomes so small, it doesn't make sense to supply it."