The Commission has levied a record fine of €750 million (£493 million) against 11 power equipment firms, of which Siemens will suffer the biggest penalty. The German electronics group is fined €396 million, a penalty that is increased by 50 per cent for its role as a ringleader in the cartel for gas-insulated switchgear (GIS).

The penalty’s size caused shares in Siemens to dip on the Frankfurt bourse, but the company said that it would appeal against the sum, which will hit its first-quarter profits. The company had made no provision for the fine.

However, ABB, of Switzerland, another conspirator, has escaped a fine altogether for blowing the whistle on the cartel to the Commission in 2004.

The Commission’s competition directorate said that between 1988 and 2004 the cartel carved up markets and rigged tender prices for GIS — equipment to control energy flows in electricity grids.

In addition to Siemens and ABB, the conspirators were Alstom, Areva and Schneider, the French groups, and five Japanese companies: Fuji, Hitachi, Mitsubishi Electric, Toshiba and Japan AE Power Systems. The switchgear suppliers informed each other of tender offers by customers, and co-ordinated bids to ensure that business was won in line with a quota for each cartel member. In addition, markets were sliced up geographically, with the Japanese agreeing not to sell in Europe, while European companies avoiding Japan.

Cartel members used high-tech methods to keep sham bids secret, the Commission said. “Code names were used for both companies and individuals . . . they relied on anonymous e-mail addresses for communication and used encryption for sending messages,” it said. Siemens supplied the encryption technology.

Neelie Kroes, the EU Competition Commissioner, said that the cartel had been cheating public utilities and consumers for more than 16 years.

ABB’s whistleblowing cut its fine from €215 million to nil. Siemens said it is to challenge its fine’s size because it was not in the cartel for all 16 years.

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