Warren Buffett’s grip on Constellation Energy appeared less certain on Wednesday after EDF, France’s state-controlled energy group, launched a final bid to preserve its strategic position in the US with a rival $4.5bn offer for 50 per cent of its partner’s nuclear assets.

The French group made the offer in spite of investor concerns about the rising costs of its nuclear investments that have hobbled its shares in recent months. In an investor presentation in London on Thursday, the group is expected to say that the oil price would have to rise past $60 a barrel to make investment in new US power stations profitable, given skyrocketing costs.