Shares in the UK's biggest nuclear power generator British Energy have fallen by more than a third after the firm said it had closed one of its reactors to probe a possible fault.

The company said on Tuesday it had closed a reactor at its power station in Torness, Scotland - which accounts for about 12% of its annual energy output - to investigate vibrations on a gas circulator.

It said it did not know when the reactor would be brought back into service, but stressed that the incident posed no danger to the public.

Investors, already worried about the impact of falling power prices on British Energy's profits, marked the firm's shares down 37% to 56 pence in early afternoon trade.

The slump in power prices - blamed in part on warmer than usual weather - is expected to push the firm into the red this year.

Circulator glitch

The latest incident means that the Torness plant is now lying idle.

The station's other reactor was shut in mid-May, and has not yet been put back into service.

That shutdown was also blamed on problems with gas circulators, used to cool the nuclear reactors down.

British Energy said it would issue a statement later on Tuesday on how the closure would affect its projected energy generation forecasts for the year.

It added that its Heysham plant - which uses similar gas circulators to those at the Torness station - was functioning normally.

British Energy also said it would carry out scheduled maintenance work on its Dungeness power plant earlier than originally planned.

The firm operates a total of eight power plants in the UK, and runs other nuclear power operations in the US.