Some of the doctors arrested over a terrorist plot to explode car bombs in London and attack Glasgow International Airport last week were already known to MI5, Britain’s domestic spy agency.

Police issued no public appeals for information and were able to round up the suspects within days, unlike in previous terrorism investigations, because their details were already on security service records, said a government official who spoke on condition of anonymity Tuesday. All had worked in the National Health Service, suggesting some British hospitals may have been penetrated by a terrorist network.

“It is my understanding that at least one of these people is on a list of 1,600 active suspects and most of the others were known to MI5,” Anthony Glees, director of the Brunel Center for Intelligence and Security, said in London late Tuesday.

Prime Minister Brown, who replaced Prime Minister Blair two days before the London car bombs were found, yesterday announced tighter rules on immigration in response to the incidents. He told Parliament his government will expand Britain’s Warnings Index, a terrorism “watch list,” to include more people and will widen the background checks of migrants entering to fill skilled jobs such as those in the medical profession.

At least four doctors of Middle Eastern origin have been arrested in connection with the plot.

The tighter security measures include “an immediate review as to what arrangements we must make in relation to recruitment to the NHS,” Mr. Brown said.

Security minister Alan West, an intelligence expert and former chief of the Royal Navy, will “examine urgently” the screening of health service workers, Mr. Brown’s spokesman, Michael Ellam, told reporters.

Employers and individuals sponsoring workers from abroad will now have to be registered with the government and undergo a background check, Mr. Ellam said. Mr. Brown also wants to share information from the Warnings Index with Arab states to create an international database of suspicious people, the spokesman said.

Police officers on June 29 dismantled two car bombs made from gas canisters, gasoline and nails parked in London’s theater and shopping district. A day later, two men rammed a Jeep Cherokee, filled with flammable material, into a terminal entrance at Glasgow airport.

British security services also had knowledge of Mohammed Siddique Khan before he led a group of suicide bombers in the July 7, 2005, attack on London’s transportation system that killed 52 people. Authorities said they didn’t put Mr. Khan under surveillance because they thought he was engaged only in raising money for Islamist causes rather than perpetrating violent acts.

A total of six men and a woman have been detained in Britain in connection with the London and Glasgow incidents. In Australia, police were given a further 48 hours to question an eighth suspect, an Indian doctor arrested there. Mohammed Haneef, 27, who practiced at a hospital in Queensland state, was detained two days ago at Brisbane International Airport as he tried to leave the country on a one-way ticket.

Mr. Haneef worked as a substitute doctor at the Halton Hospital in Runcorn, northern England, until 2005, a spokeswoman for the North Cheshire NHS Trust said yesterday. One of the men held by British police worked at the same hospital, she added.

“We don’t know yet whether the connection between this man and those arrested in Britain is malign,” Prime Minister Howard told Channel 7 television yesterday. A senior counterterrorism officer from London’s Metropolitan Police force was traveling to Australia to question Mr. Haneef, he said.

Australian Federal Police Commissioner Mick Keelty said yesterday that a second doctor, who like Mr. Haneef moved to Australia from Britain, was released without charge after questioning.

Mr. Haneef began working at the Gold Coast Hospital in Queensland in September. He was given emergency leave two days ago after telling hospital officials that his wife in India was unwell, the district health service said yesterday.