A series of alerts have accentuated jitters since the attempted bombings as Britain braces for the anniversary of the July 7, 2005 suicide attacks in London which killed 52.

All suspects so far detained are doctors or medical staff with Middle Eastern links.

According to British press reports, several of the suspects were already on the files of domestic intelligence agency MI5 as "people who knew people" that were under observation.

"It appears that there are some linkages which refer to some of the individuals that have been detained," one source said, adding that this had helped "speed up some of the investigations."

At least six of the suspects are believed to be doctors, including a Jordanian neurosurgeon and an Iraqi doctor, with one being held in Australia after attempting to leave the country.

A British counter-terrorism officer was set to arrive in Australia today to interrogate Mohammed Haneef, a 27-year-old Indian doctor who was arrested at Brisbane international airport Monday.

It was not immediately clear if Britain would seek his extradition.

Australia's Federal Police Commissioner Mick Keelty said the next action would become clearer within 48 hours.

Asked if a decision on whether Haneef would face charges, be freed or be extradited to Britain would be made soon, Keelty replied: "By the end of the week, I think, is the timeframe that we would give."

Security has been tightened across Britain since two Mercedes cars packed with gas canisters and nails were found in London's entertainment district on Friday.

The following day a blazing Jeep Cherokee rammed into the front doors of Glasgow airport's main terminal, and hours later the national security level was raised to "critical," meaning another attack is expected "imminently."

Police say the two attacks were linked and on Wednesday reports emerged that the two men who carried out the Glasgow attack may have been the drivers who abandoned the cars in London.

The first arrests in the investigation were made Saturday after police apprehended the two men driving the fiery jeep in Glasgow, one of whom remains in critical condition in hospital.

The injured man was identified by British television as Khalid Ahmed, while the other Glasgow suspect has been named as Bilal Abdulla, an Iraqi doctor.

On Sunday, a 26-year-old man and a 27-year-old woman, believed to be a Jordanian surgeon named by officials in Amman as Mohammed Jamil Abdelkader Asha, and his wife, were arrested. They are now being questioned in London.

A fifth suspect, a 26-year-old man, was arrested in Liverpool on Saturday night. On Sunday, two other men, aged 25 and 28, were detained outside Glasgow.