Scotland's electricity output was cut by a quarter yesterday when British Energy closed down both reactors at Hunterston B nuclear power station in Ayrshire.

The company said the move - due, ironically, to problems with the station's power supply - was likely to be temporary.

One of the two 485-megawatt reactors, known as unit 4, had been restarted only hours earlier following a maintenance closure but British Energy said this was a coincidence. "The two are not connected," said a spokeswoman.

Both unit 4 and its sister reactor, unit 3, had been operating at 70% of capacity since boiler problems arose last year, putting them out of action.

Yesterday's manual shut- down was decided on because of problems with the station's internal electricity supply, crucial to the plant's control system.

Last night the lights were on courtesy of ScottishPower.

"There is no reason to believe both reactors won't be coming back on very shortly," the spokeswoman said. "I would not say it was a routine occurrence but it is not unheard of."

The shutdowns come less than a week before the end of the government's public consultation on future energy needs, in particular whether to revisit the nuclear option.