The European Commission is expected today to open a full antitrust investigation into TotalFina's €49 billion ($51 billion) bid for French oil group Elf Aquitaine after the company failed to allay its concerns.

The decision by Brussels to deepen its investigation could delay clearance by up to four months. To secure approval, the companies will almost certainly have to make heavy divestments in some sectors.

The Commission decision follows a meeting last week between Mr Thierry Desmarest, TotalFina chairman, and Mr Mario Monti, EU competition commissioner, to discuss ways of reducing the companies' huge presence in France, where it will retain both the Total and Elf retail networks and brand names.

Mr Desmarest offered to move on three areas - freeway filling stations, oil pipelines and liquid petroleum gas - but his suggestions are understood to have fallen short of the Commission's demands. TotalFina said last night its offer for Elf would not be affected by the investigation.

The bid is still expected to close on October 15th, with the results published on October 27th and, assuming the bid succeeds, the exchange of shares is scheduled to start the following day.

The companies' combined board of directors and the new executive committee of senior managers will not be able to meet until Brussels approves the deal. TotalFina executives expect this early in the new year.

The EU's antitrust watchdog has become increasingly concerned about mergers that put too much power into the hands of a small number of companies. The combined TotalFina and Elf, which has been dubbed "France Oil", would become France's biggest company and the world's fourth-largest oil group, but it will still be significantly smaller than the "big three" that have emerged into a so-called superleague - Exxon Mobil, Royal Dutch/Shell and BP Amoco after its planned takeover of Arco of the US.