Consumer goods giants Unilever and Procter & Gamble have been slapped with fines totalling 315.2 million euros (£280.5 million) after they colluded to fix the price of washing machine detergent powder.
The European Commission said the two companies and rival Henkel shared information about pricing in eight countries when they worked together on an initiative to reduce the environmental impact of their products.
The Commission fined Unilever, whose UK brands include Surf, 104 million euros (£92.6 million) and P&G, which makes Ariel, 211.2 million euros (£188 million) after reductions for co-operating with the investigation.
Henkel was let off a fine after it blew the whistle to the regulator.
The three companies admitted that the cartel existed in eight European countries - Belgium, France, Germany, Greece, Italy, Portugal, Spain and The Netherlands - between January 7 2002 and March 8 2005.
They were working together on environmental measures, including a scheme to produce concentrated powder to reduce packaging, but they also shared financial information, said the EC.
A spokesman for Unilever said the company has strengthened its internal compliance programme and all key managers in Europe have been retrained on the European competition rules.