Germany’s third biggest utility EnBW has mounted a legal challenge to the government’s clearance of E.On’s bid to take over gas supplier Ruhrgas. It is the largest company to do so to date.

Earlier, a Düsseldorf court had upheld an injunction against the $10 billion buy out after a legal challenge was launched by energy firms Ampere and Trianel and the city utilities of Aachen and Rosenheim. It said that procedural errors had been made by the government when it had exercised a rarely used veto to overturn earlier cartel office objections.

EnBW’s contention is that conditions attached to the deal, including the sale of some stakes in energy companies, were not sufficient to stimulate competition.

But the government, which is sympathetic to the view that Germany needs a strong player to negotiate on foreign markets and help safeguard security of supply, cleared the deal despite fierce opposition from rivals and consumer groups.

E.On says that it will invest its E40 billion war chest abroad if the Ruhrgas deal is halted.