The Environment Agency says the problem could be felt for decades.

BP was fined £60,000 on Friday, after the 37,000 litres of unleaded fuel escaped from a 30-year old underground tank at a filling station in Guildford Street, Luton.

The tank, which is 230 metres away from the water source, was later found to be pitted, corroded and had a hole the size of a ten pence coin, according to the Environment Agency.

'Monitoring pollution'

About 140,000 litres of petrol was pumped into the tank from the time the leak was suspected last December to the time emergency teams were called in 12 days later, says the agency.

A spokeswoman said the Environment Agency was monitoring the pollution closely.

She said: "BP has tried to remove as much of 37,000 litres as it can, but it is extremely hard to remove petrol from groundwater and it is feared that the effects may be felt for decades.

"Even in small quantities, hydrocarbons in unleaded petrol are toxic and can be harmful to human health.

Water imported

"It is difficult to predict whether the pollution will reach Luton's drinking water - already it has spread 30 metres towards it."

Three Valleys Water, the operators of the Crescent Road pumping station, has been forced to reduce the amount of water abstracted by nine million litres a day, to 20 million litres.

The shortfall is being made up by importing 7.5 million litres daily from Anglian Water's Grafham Reservoir in Cambridgeshire, at a cost of £225 per day.