Unilever and Procter & Gamble have been hit with fines totalling €315.2m (£281m) for fixing the price of washing powder in eight European countries.

The European Commission imposed the penalties on Wednesday after finding that the consumer goods groups had colluded over prices for more than three years. P&G must hand over €211.2m, while Unilever will pay €104m. Both fines were reduced because the companies co-operated with the investigation and agreed to settle.

The price-fixing cartel began in January 2002, according to the commission, when P&G and Unilever, along with Germany's Henkel, held talks over plans to implement an industry-wide programme to improve the environmental performance of detergents. The companies agreed to shrink the amount of packaging they used but to keep prices unchanged, and later to collectively raise prices. The arrangement lasted until March 2005 and involved products sold in Belgium, France, Germany, Greece, Italy, Portugal, Spain and the Netherlands.

"The cartel lasted some three years and aimed at stabilising market positions and at coordinating prices in violation of EU and EEA antitrust rules," said the commission as it announced the fines.

Henkel avoided a fine after being granted full immunity for informing the commission about the cartel in 2008.

"Companies should be under no illusion that the commission will pursue its relentless fight against cartels, which extract higher prices from consumers than if companies compete fairly and on their merits," said Joaquín Almunia, vice-president of the commission, which oversees competition policy.

Unilever said in a statement that it had retrained managers across Europe so that they better understood European competition rules. P&G also said it had strengthened its global compliance programme. Both companies had made provisions to cover a likely fine from the commission.

Anglo-Dutch group Unilever makes detergent products under the brand names Omo and Surf. P&G, which is the world's largest consumer products group, owns the Tide, Gain and Era brands of washing powder. Henkel owns the Persil brand in most of Europe, although Unilever holds the rights in Britain, Ireland and France.

Procter & Gamble qualified for a 50% reduction in its original fine under leniency rules, while Unilever saw its penalty cut by 25%.

These discounts are part of the commission's attempt to speed up its investigations by using a simplified procedure.

"This is good for consumers and for taxpayers as it reduces costs; good for antitrust enforcement as it frees up resources to tackle other suspected cases; and good for the companies themselves that benefit from quicker decisions and a 10% reduction in fines," it said.

The two companies could yet face damages claims from other firms, or individuals, who claim that they suffered financial losses because of the way prices were fixed.

Unilever and P&G are the third group of companies to receive cartel-related fines from the commission in the last year. In May 2010 it hit 10 computer memory makers with fines totalling €331m. Two months later the commission fined producers of animal feed phosphates more than €175m.