French company GDF Suez (Euronext: GSZ) will have to increase its offer for 30 per cent of International Power by at least 15p per share, several people involved in the talks have said, after a leading investor in the British-listed company sold almost a third of his holding at about 405p per share.

GDF Suez already owns 70 per cent of International Power and last week made an indicative offer of 390p per share for the remainder, a bid valued at £5.95bn. But people involved in the negotiations said the sale by Neil Woodford at Invesco, a long-standing shareholder in the British company, set a new price benchmark.

 Some people working on the International Power side said GDF could be forced to pitch its offer even higher, with some mooting as much as 420p.

However, one leading investor in the company said it believed 405p was “a very fair price” for the stake. A 405p per share offer would value the stake at £6.2bn, while a deal at 420p would lift it to £6.4bn.

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