So says British intelligence:

British intelligence and law enforcement officials have passed on a grim assessment to their U.S. counterparts, “It will be a miracle if there isn’t a terror attack over the holidays in London,” a senior American law enforcement official tells ABCNews.com.
British police have been quietly carrying out a series of key arrests as they continue to track at least six active “plots” tied to what they call “al Qaeda of England.”
Officials said they could not cite any specific date or target but said al Qaeda had planned previous operations during the Christmas holidays that had been disrupted.
“It is not a matter of if there will be an attack, but how bad the attack will be,” an intelligence official told ABCNews.com.
Authorities say they are seeking at least 18 suspected suicide bombers.

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