AMP Pearl, in Peterborough, discovered a cleaner working for a car valeting company had masterminded the theft, after AMP's fleet manager raised the alarm over rocketing fuel costs.
Cleaner Andrew Jones stole petrol cards from employees of AMP Pearl as he valeted their cars. Through the firm he ran, Ace Valeting, Jones had secured a 25-month contract to clean its 60-strong fleet of high performance BMWs.
But while he cleaned the cars, Jones began to help himself to the cards which had been left in the vehicles to allow the employees to fill up with petrol.
Peterborough Crown Court heard last week that Jones then used them to get petrol for himself and his friends.
David Monk, prosecuting, told the court that Jones also sold them to taxi drivers.
With the help of his sister, Susan Hewitt and her partner, Paul Toms, the gang stole about £150,000 worth of petrol which they had not paid for.
Monk said: 'When Pearl's fleet manager became suspicious and informed the police it was discovered that the fraud had taken place between the end of 1999 and March 2001.
'They worked out that cards from 21 vehicles had been used in the scam and 18 of these vehicles were regularly cleaned by Ace Valeting.
'Police later discovered Jones made a living out of selling the cards to various local taxi drivers and by filling up cars belonging to his 'customers' at local petrol stations.'
Mr Monk added that Toms' job was to find people willing to buy either a stolen card, or cheap petrol, which would be put into their car by the gang.
Jones had pleaded guilty to one count of conspiracy to defraud at an earlier hearing along with Hewitt and Toms, who live in the Bretton area of Peterborough.
The defence said the gang had sold the fuel for much less than it was worth, making about £40,000 in total.
Judge Peter Jacobs sentenced Jones to two-and-a-half years' imprisonment, Toms to 12 months' imprisonment and Hewitt to a 200-hour community punishment order.
Fleets have been the focus of warnings that theft and misuse of fuel cards is adding to a boom in credit and fuel card fraud. Figures from the British Oil Security Syndicate (BOSS) show that last year service stations in Britain lost more than £40 million through plastic card fraud.
Fleet and fuel management company Arval PHH, which operates the AllStar fuel card, is urging fleet drivers to treat their cards with greater care to avoid offering easy pickings to criminals.
In some cases, drivers have left their cards in vehicles when they have been handed back to the supplier at the end of a supply contract, providers claim.
Card theft can be a major problem for petrol stations as some do not yet have the sophisticated anti-fraud technology that high street retailers use. Until they can upgrade their systems, fleet decision-makers need to make sure drivers are not easy targets.
Arval PHH this year won the Sixt Rent a Car-sponsored Innovation of the Year at the Fleet News Awards for its Online Authorisation system. It allows fuel cards to be immediately checked against a hotlist of lost or stolen cards before they are used to buy fuel.