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The British security official said there were similarities between the device and vehicle bombs used by insurgents in Iraq.

The official also said the domestic spy agency MI5 would examine possible connections between yesterday’s bomb attempt and at least two similar foiled plots to attack a London nightclub in 2004 and to pack limousines with gas canisters and shrapnel.

In the 2004 plot, accused members of an al Qaeda-linked terror cell were convicted of plotting to blow up the Ministry of Sound nightclub. A recording made by MI5 captured the plotters discussing an attack on the nightclub, one of London’s biggest and most famous venues.

One man is heard saying the plan was to “Blow the whole thing up.”

The discovery of the bomb resurrected fears that followed the July 7, 2005, suicide bombings that killed 52 people on three London subways and a bus and failed attacks on the transit system just two weeks later. Those attacks deepened divisions between Muslims and non-Muslims in Britain and provoked an angry debate over religious tolerance and ethnic assimilation.

Three of the suicide bombers were British-born men of Pakistani descent, and all four were Muslim. The fact that seemingly unremarkable British youths could become suicide bombers caused soul-searching across the country, and raised fears of a threat from homegrown terrorists.

Gordon Brown, who only Wednesday succeeded Tony Blair as prime minister, called it a stark reminder that Britain faces a serious and continuous threat of terrorist attacks. He urged people to be on alert.

“I will stress to the Cabinet that the vigilance must be maintained over the next few days,” he said.

The terror threat level has remained at “severe” meaning a terrorist attack is highly likely since last August.

One analyst said the bombers could be trying to send Britain’s new leader a message.

“It’s a way of testing Gordon Brown,” said Bob Ayers, a security expert at the Chatham House think tank. “It’s not too far-fetched to assume it was designed to expedite the decision on withdrawal [from Iraq].”

Yesterday morning, President Bush was briefed by National Security Adviser Stephen Hadley about the apparent terror attack. Mr. Bush is at his family’s home in Kennebunkport, Maine, where he will meet tomorrow and Monday with Russian President Vladimir Putin. The White House had no immediate comment on the incident.