BRUSSELS -- The European Union fined energy giants E.On AG of Germany and GDF Suez SA of France €1.1 billion ($1.53 billion) for agreeing not to compete on sales of natural gas in each other's home markets.

The fine -- the second highest ever assessed in an EU cartel case -- reflects the desire of the European Commission, the bloc's executive arm, to force EU-wide competition in the energy sector -- despite reluctance from some member countries.

Each...