July 4 (Bloomberg) -- 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, the U.K.'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 yesterday. All had worked in the National Health Service, suggesting some U.K. 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 yesterday.

Prime Minister Gordon Brown, who replaced Tony Blair two days before the London car bombs were found, today announced tighter rules on immigration in response to the incidents. He told Parliament his government will expand the U.K.'s Warnings Index, a terrorism ``watch list,'' to include more people and will widen the background checks of migrants entering to fill highly 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,'' Brown said.

Security Minister

Security minister Alan West, an intelligence expert and former chief of the Royal Navy, will ``examine urgently'' the screening of health service workers, 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, Ellam said. 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.

U.K. 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 Khan under surveillance because they thought he was engaged only in raising money for Islamist causes rather than perpetrating violent acts.

Eight Suspects

A total of six men and a woman have been detained in the U.K. 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.

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 John Howard told Channel Seven television today. A senior counterterrorism officer from London's Metropolitan Police force was traveling to Australia to question Haneef, he said.

Doctor Released

Australian Federal Police Commissioner Mick Keelty said today that a second doctor, who like Haneef moved to Australia from the U.K., was released without charge after questioning.

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 today.

``He is innocent,'' said Haneef's mother, Qurrathunain, 48, in an interview at her home in the Indian city of Bangalore today. ``He is being targeted because he is a Muslim.''

She said Haneef's wife gave birth to a baby girl 10 days ago and that the child is sick with jaundice. ``He should have come home yesterday,'' his mother added.

Six of the suspects in the U.K. are being questioned at the high-security Paddington Green police station in London. The seventh set himself on fire in the Glasgow attack and is in critical condition at a hospital in Scotland.

Threat Level Lowered

The U.K. today reduced its terrorist-threat level to ``severe'' from ``critical,'' Home Secretary Jacqui Smith said today in an e-mailed statement.

The level was raised to ``critical,'' the highest on a five- tier scale, after the London car bombs were found. ``Critical'' means a terrorist attack is expected imminently, while ``severe'' means intelligence officials believe an attack is highly likely.

``There is no intelligence to suggest that an attack is expected imminently,'' Smith said in the statement.

The threat level is set by the Joint Terrorism Analysis Centre, created in 2003 and based at MI5's London headquarters.

A leader of the group al-Qaeda in Iraq warned Canon Andrew White, an Anglican minister working in Baghdad, in April of attacks on U.K. targets and suggested they could be carried out by medics, the cleric told British Broadcasting Corp. television in a telephone interview today.

`Cure' and `Kill'

White said he met the man at a conference on religious reconciliation in the Jordanian capital, Amman. ``He said the people who cure you will kill you,'' White told the BBC.

The cleric said he told the U.K. Foreign Office of the threat of attacks without relaying the comment that he now links to the doctors who were arrested. ``I did not realize at the time how significant that was,'' he told the BBC.

The Foreign Office today acknowledged receiving information from White about the Amman meeting, adding that it was considered at the time to be too vague to merit further analysis. White's information has since been passed on to police investigating the Glasgow and London incidents, a Foreign Office spokesman said.

Brown's government has stepped up security in the U.K. Controls were tightened at airports and more police are patrolling public areas, including London's two financial districts, the City and Canary Wharf.

Brown has spoken to U.S. President George W. Bush, North Atlantic Treaty Organization Secretary General Jaap de Hoop Scheffer and Indian Prime Minister Manmohan Singh in the last few days about the terrorist plot in the U.K., Ellam said.

To contact the reporters on this story: Nick Allen in London at nallen14@bloomberg.net; Ed Johnson in Sydney at ejohnson28@bloomberg.net.

Last Updated: July 4, 2007 13:01 EDT