BRUSSELS (Reuters) - The European Commission has opened an investigation into the award of a Eurostar train order to German group Siemens after a complaint by French company Alstom, an EU source said.

The EU executive began a preliminary four-week investigation at the end of October into objections by Alstom to the contract for trains to run through the Anglo-French Channel Tunnel, the source said yesterday.

"Alstom have raised questions which do justify further investigation," the source said. "But the Commission is not pre-judging anything. It is far too early to say. As of today, the contract has not been signed."

The only route available to the Commission at this stage would be standard infringement procedures against Britain for not respecting EU rules on public procurement, the source said.

A row erupted when Eurostar, which runs passenger services between London, Paris and Brussels, said Siemens would supply 10 next-generation high-speed trains as part of a £700 million ($1.13 billion) fleet investment, a snub to Alstom, its traditional supplier.

The French government has said the train deal is invalid, citing safety concerns and pointing to rules forbidding the use of a traction system such as that under the carriages of Siemens trains.

The European Commission said on Wednesday it was awaiting a response from Britain to Alstom's charges that the order broke EU public procurement rules.

The Commission had said last month said that on the basis of information it had received from Alstom it could not find fault with the way Eurostar picked Siemens for the deal.

It also considered that an ongoing review of Channel Tunnel safety regulations was not an EU matter and that the Anglo-French Intergovernmental Commission (IGC) which governs the operation of the tunnel, was responsible for this.

Last week, a British court turned down Alstom's application to block the deal, but the company vowed to keep up its legal fight, in Britain and at the European level.

"I have a simple aim, and that is for the invitation for tenders to be halted and on the basis of a finalised set of regulations and a clear and transparent set of rules, we will be able to respond to the call for tenders," said Alstom chief executive Patrick Kron.

"It is nothing to do with protectionism," he said, speaking at a news conference on the group's first half results.