UPDATE 2-British Energy's Hunterston B7 reactor shuts

(Adds comment from British Energy about trip, background)

LONDON, May 28 (Reuters) - British Energy's BGY.L 595-megawatt Hunterston B7 nuclear power reactor in Scotland shut down unexpectedly on Wednesday afternoon, the company said.

The reactor tripped at around 1345 GMT but British Energy was unable to say what had caused it.

"It was an unplanned trip and at the moment they are investigating the cause," a spokeswoman for Britain's largest nuclear power company said, adding that it was too early to say how long the plant might be closed for.

The stoppage came after unexpected shutdowns at the company's Sizewell B power plant in England and Scottish Power's (IBE.MC) Longannet 1 coal fired plant triggered brief power cuts in parts of Britain on Tuesday.

The closure of Hunterston B7 meant 10 of the company's 16 nuclear generation units were not working on Wednesday afternoon.

National Grid said UK power supplies were comfortable on Wednesday afternoon because other plants had been brought online since it pleaded for more generation to be made available on Tuesday.

"We are still happy with the supply margins," a spokesman for National Grid said.

British Energy chief executive Bill Coley said on Wednesday morning that the company expected to restart the 1,180-megawatt Sizewell B power station within a few days.

(Reporting by Daniel Fineren)