EDF, the nuclear power operator, has bid $4.5 billion (£3 billion) for a half-share in Constellation Energy of America, in an attempt to frustrate a takeover offer by Warren Buffett, the bil-lionaire investor.

The French state-controlled energy group has offered the cash in exchange for a 50 per cent stake in Constellation’s nuclear operations, having backed down from a full takeover bid in October.

Constellation is a leading operator in the nuclear power market in the United States and is viewed by EDF, the world’s biggest nuclear power operator, as a high strategic priority for its international expansion plans.

EDF, which already owns 9.5 per cent of the shares in Constellation, is already committed to a £12.5 billion takeover of British Energy, the UK nuclear generator.

EDF has offered $52 a share for the Constellation stake, representing a 96 per cent premium to an earlier $26.50-per-share offer tabled by Mr Buffett’s MidAmerican Energy Holdings.

Shares in EDF fell by 6 per cent after the announcement, amid fears that the French company was overextending itself with two large international investment plans at the same time. However, the stock recovered to close almost unchanged at €44.54.

Constellation operates three nuclear power plants containing five reactors in Maryland and New York, as well as other conventional power stations. Nuclear power represents 61 per cent of its total generating capacity of 8,700 megawatts. However, Constellation is in financial difficulties and needs a $1 billion cash injection to avoid bankruptcy.

EDF, which pulled out of its $35-a-share offer for the whole of Constellation, has offered this infusion as part of its bid. EDF is already in business with Constellation, after an agreement in July 2007 to help it to build two French-designed nuclear reactors.

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