Speculation that a major new gas field has been found in the North Sea is believed to be behind what could be a £1.25 billion hostile takeover of Venture Production by Centrica.

British Gas group Centrica earlier this week swooped to buy 22% of specialist North Sea developer Venture at a cost of £240 million. Venture's shares have since soared through the 725p that Centrica paid, and were today at 755p.

The shares are being talked up as some traders believe Centrica has launched its bid because of news from Venture's drilling programme in the southern North Sea, especially in its Cygnus field. Cygnus is 48.75% owned by Venture alongside the field's operator, GDF Suez of France.

GDF Suez last month drilled an appraisal well, where initial results indicated Cygnus could be one of the most important finds in the North Sea in years, in an oil and gas province that is universally recognised as being in long-term decline.

Venture chief executive Mike Wagstaff has been telling investors: "Our belief is that the Cygnus field is one of the southern North Sea's largest undeveloped gas fields."

Venture bought its stake in Cygnus last year from Tullow Oil, the FTSE 100 explorer that has switched its focus to Africa, and German giant E.ON Ruhrgas.

Venture today said it has made another gas discovery in nearby North Sea field Carna.