Britain's top security official is warning that it is highly likely militants will try to mount an attack in the country over the holiday season.

Home Secretary John Reid said Sunday 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, who was in London, 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.

Airports in Britain, Canada, the United States and elsewhere boosted security in August after police in London said they had arrested two dozen people under anti-terrorism laws.

They alleged that the suspects had been within days of trying to blow up as many as 10 commercial flights.

In November, the head of Britain's MI-5 intelligence agency, Dame Eliza Manningham-Buller, said authorities in the country were tracking almost 30 terrorist plots involving 1,600 individuals.

She said many of the suspects were homegrown British terrorists plotting suicide attacks and other mass-casualty bombings.

Manningham-Buller also said MI-5 had foiled five major plots since the July 2005 transit bomb attacks in London, which killed 56 people, including the four attackers.