The long-running bid process for British Energy has suffered a fresh blow after it emerged that the group was forced to shut down one of its largest power-producing units.

Unit 1 of British Energy’s Torness reactor was taken out on Saturday for an unspecified length of time, highlighting the company’s ageing, problematic infrastructure.

The shutdown is a setback because it raises questions over the valuation of British Energy’s assets and the cost of keeping them in operation until they are retired.

The problems at the reactor, near East Lothian, Scotland, come after a six-week outage in December and January and another week-long outage at Unit 2 in February.

The energy company is playing off Europe’s sector heavyweights one against the other to secure the highest price. Last week EDF, of France, and RWE, of Germany, emerged as the front-runners to buy the business, with RWE thought to have submitted an indicative all-cash offer of nearly 700p a share several weeks ago, valuing it at £11 billion.

However, a banking source has since told The Times that RWE’s bid was “substantially” below that level. A spokesman for British Energy refused to disclose the nature of the problem at Torness or when it would be fixed, but said that the company was “looking at a restart plan”.

British Energy, whose eight nuclear plants and one coal-fired facility produce about one sixth of the UK’s electricity, is likely to update investors on the state of Torness and problems at other reactors at Heysham and Hartlepool when it holds an investor day today.

While British Energy generates about 10gW of electrical power, most of its reactors are scheduled to be retired within a decade.

Several have technical problems, including cracks in boilers and pipes. Most of the company’s value to a new bidder is tied up in the sites for new power plants.

Last week British Energy reported an annual fall in nuclear output because of plants being taken offline. The generator, which is 35 per cent-owned by the Government, is viewed as a key player in the drive to build a new fleet of British reactors through its ownership of the preferred sites – including Dungeness, Hinkley Point, Sizewell and Bradwell.

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