With eight suspects in a pair of failed car bombings now in police custody, Britain on Wednesday lowered its national threat level, while still urging the public to remain vigilant.

British Home Secretary, Jaqui Smith, said the decision to lower the threat from “critical” to “severe” had come after the authorities concluded that there was “no intelligence to suggest that an attack is expected imminently.” But in a statement, she said, “there remains a serious and real threat against the United Kingdom, and I would ask again that the public remain vigilant.”

While the lowering of the threat level may soothe nerves here, people are shaken by the disclosure that the suspects hatched their plot while working in Britain under the umbrella of the National Health Service.

Britain will conduct a thorough review of its policies for screening foreign doctors who want to practice here, Prime Minister Gordon Brown announced Wednesday. Brown, speaking in Parliament in his first appearance at weekly Prime Minister’s Questions, also said the government would also expand its worldwide watch list of potential terrorists.

“It is vitally important the message is sent out to the rest of the world that we will stand strong, steadfast, and united in the face of terror,” said Brown, who faced challenging questions from the Conservative opposition about his government’s treatment of radical Islamic groups.

Five days after a failed attempt to blow up cars in London and at Glasgow Airport, the police are confident that all the main suspects in the case have been rounded up, a law enforcement official said, speaking on condition of anonymity.

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