Security experts were considering reducing Britain's "critical" terrorist threat level on Wednesday as a police source said they believed the main people suspected of an Al-Qaeda style bomb plot had been arrested.
Britain ramped up its security alert level to its highest level, "critical," after two car bombs primed to explode were found in central London on Friday and a fuel-packed jeep was rammed in Glasgow airport in Scotland on Saturday.
Eight people have been arrested, including one in Australia. Many of the suspects are doctors or have medical links.
The "critical" level is necessarily short term because it assumes an attack is imminent. A police source said detectives believed they had now the main potential attackers in this plot but were still looking for other possible members of the cell.
A government spokesman said security experts from Britain's Joint Terrorism Analysis Centre keep the threat level under constant review. If it is eased, it is likely to be brought down just one notch, to severe. "As soon as there is an announcement it will be made public," he said.
Police said a British counter-terrorism officer was en route to Australia to help detectives there question an Indian doctor detained on Tuesday in connection with the plot.
A police spokesman said detectives were continuing to question six people being held in London. A seventh man arrested in Scotland after a fuel-laden jeep rammed into Glasgow airport remains critically ill in hospital with severe burns.
A security source told Reuters the names of some of the suspects were on an MI5 intelligence agency database, but they were not under surveillance at the time of the attacks.
A counter-terrorism official in Washington said none of the suspects arrested was named on US terror alert watch lists.
Police declined comment on media reports the same two people were responsible for the attempted London and Scottish bombings. They also declined to say whether they expected more arrests.
Security analyst MJ Gohel of the Asia-Pacific Foundation in London said the threat level could be reduced in the next 24 hours. "They seem to have now identified virtually every person of interest to them," he said.
Security sources said two of the suspects were Indian, the rest were Middle Eastern and "quite a few" were doctors -- a contrast with recent British conspiracies led by "home-grown" militants, often with modest academic backgrounds.
Two of those arrested worked at hospitals in England, one was a doctor in Scotland, and the doctor detained in Australia, Mohamed Haneef, is an Indian national who had worked in Britain. Police sources said the other suspects also had medical links.
Of the other doctors held over the plot, British police sources named one as Bilal Abdulla, who qualified in Iraq in 2004, and another as Mohammed Asha, a Jordanian who qualified in the same year. Asha's wife was also arrested.
The Times newspaper said a senior British Anglican cleric in Baghdad had received a warning from an Al-Qaeda leader in Iraq in April that attacks were planned in Britain and America and that "the people who cure you would kill you."
Canon Andrew White told the Times he had passed the warning on to officials at the foreign office.
Britain has seen a marked increase in terrorism-related plots since the September 11 strikes on the United States and its decision to join US forces in invading Iraq in 2003.
The attacks pose a difficult test for Prime Minister Gordon Brown, a Scot who replaced Tony Blair last week and has come under some pressure to withdraw British troops from Iraq.
A security source said it was too early to identify a ringleader, adding: "We don't know enough to say whether they were radicalised here or overseas, or how they met."