LONDON (AP) - It's highly likely that terrorists will attempt to mount an attack in Britain over the holiday period, the country's top security official said Sunday.

Home Secretary John Reid said the threat of an attack in Britain remains severe, but gave no details of possible plots.

"The threat in this country is very high indeed," Reid told the television program GMTV. "We know that the number of conspiracies of a major type are in the tens - 30 or round about that."

The government's threat status was raised to "severe" - the second-highest level - in August, when an alleged plot to down transatlantic jetliners with liquid explosives was uncovered. Reid said the level meant "it is highly likely there will be a terrorist attempt."

Reid said he did not believe an attack was inevitable, but added terrorists only had to get through once.

"Our security services have to be successful on every occasion to prevent that happening," Reid said.

Copyright 2006 Canadian Press