The terrorist group behind the latest wave of bombing plots has not yet been neutralised and other attacks could hit cities in the United Kingdom, security sources told The Times yesterday.

As the head of Scotland Yard’s Counter Terrorism Command confirmed that the two car bombs discovered in London and the blazing Jeep incident at Glasgow airport were linked, a source said: “There is a group of individuals out there who have the capability and the intent to carry out attacks in the UK.

“In our judgment it is very likely there will be further attacks.”

The alert status was raised to “critical” at the weekend and will stay there until MI5 and the police are sure that there are no further attacks being planned by the cell. Those responsible for parking two bomb-primed cars in the West End are still on the run.

Commuters and people travelling longer distances by rail and air will notice changes today with a heightened police presence. A Scotland Yard spokesman said that more officers would be on patrol at mainline stations and on the streets to act as a “highly visible reassurance”.

Peter Clarke, Scotland Yard’s counterterrorism chief, said he was confident that the investigation would uncover who was behind the attacks. Last night there were five people in custody, one of them a 27-year-old woman, all arrested in connection with the plots. The woman was arrested with a man after police blocked their car while travelling on the M6.

Of the five, two are understood to be doctors, while none is thought to be a British citizen.

The driver of the Jeep Cherokee that crashed into the terminal at Glasgow airport suffered severe burns after setting himself alight and was still in hospital in a critical condition. To add to the drama, police carried out a controlled explosion on a vehicle in the car park of the hospital where the suspect was being treated. Strathclyde Police said that the car was connected to the airport attack.

MI5 and the police are searching through their databases to see if there is any trace of known suspects who may be connected to the London and Glasgow plots. One source said that the real fear was that the terrorists may turn out to be new suspects who had evaded MI5’s expanding surveillance and monitoring operations — known as “clean skins”.

In his first broadcast interview since becoming Prime Minister, Gordon Brown said that the attacks in London and Glasgow were perpetrated by people who were associated with al-Qaeda. Jacqui Smith, the Home Secretary, chaired a meeting of the Cobra emergency committee yesterday and will make a statement to the Commons today.

Mr Brown urged people to continue “living their lives as normal”. But for the general public, the repercussions of the plots were immediately clear. Tighter restrictions at airports caused long queues at the start of the holiday season. There was also a heavier police presence at railway stations.

Reports that a number of terror suspects on control orders who had absconded might be responsible were dismissed by security sources. Seven suspects have gone missing, but it is thought they have all left the country. Sources also denied a report from the US that America had warned British intelligence two weeks ago of a plot to attack Glasgow airport.

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