Motorists were last night warned to expect petrol prices to rise above £4 a gallon after the cost of oil moved close to $50 a barrel.

Attacks in Iraq overnight sent the price of a barrel of oil to a record high of $48.98.

The average cost of a gallon of unleaded petrol is currently £3.71, or 81.7p a litre, meaning the annual cost of filling the tank is £150 higher than in January.

The AA motoring organisation said motorists faced a roller-coaster ride over the next few months as rising interest rates compounded the cost of crude.

There also remained the threat of an increase in fuel duty in November, although Chancellor Gordon Brown could postpone an increase.

AA spokeswoman Rebecca Rees said: "Analysts are predicting it could hit £4 a gallon or 88p a litre and we have no reason to dispute that.''

Concerns about supplies have intensified after violence flared in Iraq, where militants threatened to set oilfields alight in retaliation for a US assault in the city of Najaf. Also, Opec is pumping oil at close to full capacity.

Saudi Arabia's announcement last week that it was able to pump an extra 1.3 million barrels a day had only a short-term effect in restraining prices.