Security experts are considering relaxing the official estimate of the terrorist threat to the UK from "critical" to "severe".

The move comes as six people arrested over failed car bombings in Glasgow and London are being questioned at London's Paddington Green police station.

A seventh man, Khalid Ahmed, remains in hospital after the Glasgow attack.

An eighth man, Dr Mohammed Haneef, continues to be questioned in Australia after his arrest at Brisbane Airport.

Seven of those arrested are believed to be doctors or medical students while one formerly worked as a laboratory technician.

A green Jeep Cherokee loaded with gas cylinders crashed into the doors of Glasgow Airport's main terminal and burst into flames on Saturday afternoon.

The previous day two Mercedes containing petrol, gas cylinders and nails were found outside a nightclub in London's Haymarket and at a vehicle pound after being towed from a nearby street.

On Tuesday night, a second doctor who was being questioned in Australia was released without charge.

The Metropolitan Police have said a counter-terrorism officer is travelling to Australia to liaise with authorities.

BBC correspondent Danny Shaw said the first phase of the investigation - rounding up suspects to prevent further attacks - was drawing to a close.

Security experts are now considering a relaxation of the official estimate of the terrorist threat to Britain, currently judged to be critical.

If the Joint Terrorism Analysis Centre concludes that another attack is no longer imminent, the threat level will return to severe - one level below critical.

That would mean a slight reduction in security procedures and controls.

Investigators are now focusing on analysing evidence and interviewing suspects, our correspondent said.

Dame Pauline Neville-Jones, the former head of the Joint Intelligence Committee and the new shadow security minister, said terrorists will often target places like nightclubs - where the London bombs were left.

"They take the view that people, and women in particular, who engage in that kind of activity and go around, as they would regard it, half naked, are themselves not any longer victims.

"They are actually associated with and part of the kind of world that these people have decided they must combat."

NHS link

Security sources have told the BBC that none of the suspects has been under surveillance or the subject of an anti-terrorism operation before.

But details of some of them were on an MI5 intelligence database because of their alleged links with other individuals or inquiries.

On Tuesday, controlled explosions were carried out on a car at a mosque in Glasgow and on three fire extinguishers on a pavement in Hammersmith, west London.

Later, the departure lounge at Heathrow Airport's Terminal 4 was partially evacuated after a suspect bag sparked a security alert.

Also on Tuesday, it emerged that seven of the eight people arrested were doctors or medical students.

All worked in NHS hospitals.

Sian Thomas, deputy director of NHS Employers, said she wanted to reassure the public there were "thorough and robust checks" in place before doctors were employed by NHS trusts.