Britain's competition authorities were put on alert today after German power giant RWE tabled an £11 billion takeover offer for British Energy, the UK's nuclear generating company.

The cash offer at just under 700p a share from RWE, which trades in Britain as npower, came as the heat is turned up on the future of British Energy, which runs the UK's eight main operational nuclear power stations and which accounts for nearly 20% of the nation's electricity.

British Energy is key to the future multi-billion-pound spending programme on new nuclear power stations which are likely to be located at or next to the existing sites of British Energy's reactors.

It had been assumed British Energy would be bought up in a joint venture potentially involving all of RWE, EDF of France and the UK's largest household energy company Centrica, the name behind British Gas.

An outright takeover of British Energy by RWE would spark an investigation by UK competition authorities as the combined company would control around a third of the country's electricity generating capacity. When RWE bought npower at the turn of the decade it took control of its generating fleet including larger coal and gas-fired plant at major installations close to London at Didcot and Little Barford.

RWE has around 8000 megawatts of generating plant in the UK and British Energy more than 12,000MW.

Neither RWE nor British Energy would comment on the offer.

Separately, British Energy admitted today its ageing power stations are continuing to play up. It said output reductions from planned refuelling operations will be a third higher than expected because of problems at its Dungeness plant in Kent.