Almost 100 "pop-up" petrol stations have been uncovered in Northern Ireland in the last year.

In Belfast, temporary illegal stations have been selling laundered red diesel for as little as 90p a litre.

Accounts found at one such station showed it was making £22,000 a month.

Red diesel is intended for farm or building use but various criminal gangs across Northern Ireland bleach and filter it before selling it illegally and cheaply for profit.

The information has been uncovered as part of a BBC Panorama investigation into illegal fuel operations in the UK.

Figures from HM Revenue and Customs (HMRC) for 2010/11 show widespread diesel laundering and illegal sales across Northern Ireland.

With 60% of pump prices down to taxation there is a plenty of room for the black market to profit from drivers.

HMRC also said there is some evidence that organised criminals from Northern Ireland are attempting to shift their business to the rest of the UK.

Vehicles taxed for the roads are not allowed to run on red diesel.

Cars found running on illegal fuel, such as cleaned red diesel, can be seized by customs officers or their owners subjected to a £500 on-the-spot fine.

Moral question

Also, the HMRC warns that the chemicals being used to strip the red dye out of the diesel can badly damage vehicle engines.

DUP MP Gregory Campbell, a former member of the Northern Ireland Affairs Committee who has spent years monitoring organised crime and paramilitary activity, said cheap illegal fuel comes with a moral trade-off.

Mr Campbell said in many cases the big players in Northern Ireland-based fuel crime are former paramilitaries who have graduated from funding terrorism during the Troubles to become profiteering fuel gangsters.

"The type of people who are probably involved at the top of this are the terrorists of yester-year who would like to be the oil barons of tomorrow," he said.

Quentin Willson, of the Fair Fuel UK Campaign group, said the price of petrol has reached the point where it is straining motorists' moral compass, helping to fuel the criminal activity.

Mr Willson said people, especially small business owners, are having to choose between being law-abiding and not being tempted by bargain-priced fuel that they know is likely stolen and avoiding bankruptcy.

"To be good...and law-abiding and go bankrupt or to steal fuel, and I'm not for a minute condoning that, but to be in such a situation I have to say, I've got to steal fuel to keep my business going."

The effect on business of fuel prices that have risen by a third in the past three years is borne out by research conducted for the transport industry that found that 75% of companies that instructed insolvency in 2010/11 cited the cost of fuel as the main reason for their situation.

Panorama: The Great Fuel Robbery, BBC One, Monday, 17 October at 20:30 BST and then available in the UK on the BBC iPlayer.