Offering corporate whistle-blowers full or partial immunity from fines for anti-competitive behaviour is one of the key weapons in the European Commission’s high profile campaign against illegal cartels.

The lucrative EU incentive is not new. It was first introduced 12 years ago in a leniency policy which has since been refined twice.

But it has come to the fore in the past three years as Neelie Kroes, the Competition Commissioner, has stepped up efforts to uncover secret agreements between companies.

For her, such behaviour prevents fair competition from operating and penalises customers who have to pay higher prices while being offered less choice, and can be best be tackled by a carrot and stick approach.

Ms Kroes said: “Secret cartels undermine healthy economic activity. To root out cartels, we need heavy sanctions to deter cartels and an efficient leniency policy providing incentives to report them.”

The policy has been so successful that almost three-quarters of the successful cartel investigations carried out by the Commission have relied on a company coming forward, putting its hands up and spilling the beans on its former partners.

BP was one of the recent beneficiaries of the incentive.

Last autumn, it escaped a €66 million (£50.4 million) fine after blowing the whistle on an illegal bitumen cartel in Spain.

Over a 12-year period, BP and four companies – two Spanish and one each Swedish and Portuguese – had met around an “asphalt table” to carve up the Spanish market and fix prices for bitumen.

BP’s confession provided the Brussels authorities with the information they need to bust the cartel. While its potential fine was totally expunged, the other companies were given penalties ranging from some € 9 million to just almost € 84 million.

The revisions made to the leniency policy two years ago are designed to clarify the exact information which an applicant must provide in order to successfully benefit from immunity.

They introduce flexibility as to when a company decides to stop participating in the cartel and make clear that no relevant information should be destroyed or falsified while a company is considering lifting the lid on the illegal activities.

The Commission emphasises that evidence which requires little or no corroboration has the greatest value and will lead to the largest reduction in potential fines.

While an initial whistle blower can expect to receive full immunity, other companies which cooperate fully during the investigation may find their fines reduced. On the other hand, penalties are increased for repeat offenders which have already been found guilty of operating a cartel.

In addition, the Commission’s procedures are designed to ensure that any statements companies make when providing information to EU investigators about their earlier activities may not be used against them in any subsequent civil damage procedures.

The Commission has no incentive scheme for individuals who come forward with evidence of wrongdoing. Instead, it offers them total immunity, refusing even to say whether it has ever received information from such sources.

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