Britain’s concern over Mr. Awlaki and his group rose sharply on Wednesday with two developments. A young woman who had embraced his cause and watched dozens of hours of his videos was sentenced to life in prison for the attempted murder in May of a prominent legislator, and Theresa May, home secretary in the government of Prime Minister David Cameron, announced that a member of the Yemeni Qaeda group had been arrested earlier in the year in a previously undisclosed bombing plot against the country.

British officials had warned with increasing urgency of the hazards of allowing the al-Awlaki videos to remain posted on YouTube.

Perhaps the starkest warning came in a speech delivered last month to a private audience at the Brookings Institution in Washington, reported by The Daily Telegraph on its Web site on Wednesday. Pauline Neville-Jones, a former high-ranking diplomat who is security minister in the Cameron government, said of the videos that Britain would “take them down” if it was purely a British issue, but that the implications were “global” and required action by the United States.

“These Web sites would categorically not be allowed in the U.K.,” she said. “They incite cold-blooded murder, and as such are surely contrary to the public good.”

Britain’s security agencies have wrestled with dozens of terrorist plots in recent years, successfully foiling most but suffering deeply from the attack on the London transit system in July 2005, which left 56 people, including four suicide bombers, dead.

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In recent months, top security officials here have issued a series of warnings, saying that an increasingly dire threat came from groups inspired by Osama bin Laden based in Yemen, Somalia and North Africa.

Jonathan Evans, chief of Britain’s domestic intelligence agency, MI5, said recently that Mr. Awlaki and Al Qaeda in the Arabian Peninsula were “of particular concern” in the light of their role in the attempted bombing on Dec. 25 of an American trans-Atlantic airliner approaching Detroit, and “because he preaches and teaches in the English language, which makes his message easier to access and understand for Western audiences.”

Scotland Yard detectives who investigated the attack on the legislator said outside the court that 21-year-old Roshonara Choudhury, a theology student, watched YouTube videos that showed sequences from sermons by Mr. Awlaki in Yemen in which the preacher urged Muslims everywhere to join in a worldwide holy war against the West. In a transcript of her interrogation published by The Guardian, she spoke of watching hundreds of hours of his videos. She said her motive was to “punish” the legislator, Stephen Timms, for voting in 2003 for Britain’s participation in the invasion of Iraq.

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Her lawyer told the court that Ms. Choudhury, whose parents immigrated to England from Bangladesh, had been a Muslim moderate “of exemplary character” with no links to terrorist groups until she began browsing militant Muslim Web sites.

When she attacked, as Mr. Timms met with constituents at his office in a London suburb, she was wearing a black floor-length gown and a head covering that revealed only her eyes. She pulled out a knife and stabbed him twice in the abdomen.

Ms. Choudhury refused to attend the trial, saying she did not recognize the legitimacy of the British court system. But she appeared by video link from a prison in London for Wednesday’s sentencing, when the judge, Sir Jeremy Cooke, said that she would have to serve a minimum of 15 years before applying for parole. He described Ms. Choudhury as “an intelligent young woman who has absorbed immoral ideas and wrong patterns of thinking,” and added: “You do not suffer from any mental disease. You have simply committed evil acts coolly and deliberately.”

YouTube has faced other periods of pressure to remove videos linked to radical Islamists. Jeffrey Rosen, a professor of law at George Washington University who has written extensively about YouTube’s policies, including in The New York Times Magazine, said that in 2007, the Labour Government in Britain called on YouTube to block terrorist recruitment videos featuring Islamic fighters with guns and rockets.

Last May, Senator Joseph I. Lieberman’s staff asked Google to remove about 120 terrorist recruitment videos from YouTube. Google removed some videos that showed gratuitous violence or hate speech, but refused to take down others.

“YouTube and Google deserve credit for trying to distinguish videos that are merely offensive from those that show graphic violence or hate speech or risk inciting imminent violence, which is the line American courts have drawn in free speech cases since the 1960s,” Professor Rosen said.