European Commissioner Mario Monti has repeated his warning to carmakers not to violate European competition laws as he considers a review of the existing Block Exemption regulations.

Last week the Commission fined Opel Netherlands £25m for deliberately obstructing customers from other EU states who were attempting to buy cheaper cars in Holland. The GM subsidiary may appeal against the fine, the fifth highest imposed on one company for breaking EU law.

Mr Monti said he would consider “abuses of the Highway Code of car distribution” when he presents his assessment of Block Exemption by year end.

“The measures applied by Opel Netherlands were destined to prevent consumers taking advantage of the benefits of the single market,” said Mr Monti. “The company should have known its behaviour was incompatible with the European competition rules and, in particular, with the Block Exemption regulation.”

The European Commission found that Opel Netherlands had instructed dealers close to its borders not to sell cars for export. For almost 18 months, it had operated a sales campaign where bonus payments were refused if dealers carried out sales to customers from abroad.

The Opel judgment follows a similar case in 1998 when Volkswagen was fined £50m for restricting sales from Italy.