The suspects include one doctor from Iraq and two from India. There is a doctor from Lebanon and a Jordanian doctor and his medical assistant wife. Another doctor and a medical student are thought to be from the Middle East.
Two more men were arrested in the north of England after fuel canisters were delivered to an industrial estate.
No one has been charged in the car bomb plot. Investigators believe the main plotters have been rounded up, including one in custody in Australia.
Police are still hunting for others involved in the periphery including at least one British-born suspect, a British government security official said on condition of anonymity.
The official said some of the detained suspects were found in Britain's domestic spy agency MI5's databases.
"There is suspicion, there is a complex investigation under way. But we should be cautious here. Dr Haneef may have done nothing wrong, and may at the end of the day be free to go"
Mick Keelty, Australian federal police
Meanwhile, a senior British investigator was travelling to Australia on Wednesday to question the Indian doctor arrested there in connection with the failed UK bombings.
Australian police arrested Muhammad Haneef, 27, at the international airport in the eastern city of Brisbane on Monday as he tried to leave the country.
Officials said they acted on information provided by their British counterparts.
Mick Keelty, the Australian federal police commissioner, said Haneef, 27, was being detained without charge under counterterrorism laws, as long as a judge agrees there are grounds to do so.
Keelty said a second doctor at the hospital, who was questioned by authorities because of information divulged by Haneef, had been released without charge.
No charges have been filed against Haneef, and Keelty stressed that his role, if any, in the plots in Britain was far from established.
"There is suspicion, there is a complex investigation under way," he said. "But we should be cautious here. Dr Haneef may have done nothing wrong, and may at the end of the day be free to go."
John Howard, the Australian prime minister, refused to say whether British authorities had sought Haneef's extradition.
He said there was no evidence an attack was being planned in Australia.