The European Union's competition watchdog slapped its fourth-biggest ever fine on five European chemical firms Wednesday for violating EU competitions rules by running an industrial plastics cartel.

The European Commission found Arkema (formerly Atofina), Degussa, ICI, Lucite and Quinn Barlo (formerly Barlo) guilty of running a cartel on the market for acrylic glass.

As a consequence it levied a 344.5 million euro fine on them for price fixing which it said continued between 1997 and 2002.

"Cartels are a scourge. I will ensure that cartels will continue to be tracked down, and punished," said EU Competition Commissioner Neelie Kroes, adding she was "shocked" that firms like ICI and Arkema had been fined again.

Arkema and ICI had their fines increased by 50 percent as they are repeat offenders.

"These fines will serve as a cold shower for the management and shareholders of all these companies, who have to realise that cartels cannot and will not be tolerated," she added.