Indebted nuclear power firm British Energy, surviving on a state loan, said yesterday its first-half loss ballooned to £234 million and its financial and trading prospects remained "challenging".

Britain's biggest nuclear power producer, which provides about a fifth of the country's electricity, said its loss from continuing operations for the six months to September 30 was before tax and exceptional items and compared with a loss of £60 million in the same period last year.

The firm said it had suffered a number of unplanned outages and delays to the return of services at two of its plants. It also contracted to sell much of its planned nuclear output for the current financial year during the previous year, and so has not seen the full benefit of recent electricity price rises.

"These factors, as well as increased pension costs and an increased depreciation charge related to the impairment reversal made in March 2004 have had a significant adverse impact on our profitability and cash flow," British Energy said in a statement.

"In view of the recent unplanned outages and the delayed return to service of Hartlepool and Heysham 1, the directors consider that the outlook for the company's financial and trading prospects for the remainder of the financial year will be challenging."

British Energy nearly collapsed two years ago after weak power prices forced its high-cost operations towards insolvency.

The company de-listed its shares in October in a move to fend off shareholders who stand to lose while creditors gain from a rescue plan for the company.

Creditors agreed a deal last year with the company and the UK government, giving them 92.5 per cent of undiluted equity in return for cancelling more than £1.3 billion debt.