Prime Minister Gordon Brown has said Britain will not bow down to the continued threat of terrorism, as police arrest five people in connection with three thwarted attacks in the UK this week.

The new British premier addressed the media after a third attack saw a blazing car driven into the entrance of Glasgow Airport's terminal building in Scotland. The police have now said that the Glasgow attack is linked to the two attempted car bombs that were successfully defused in London in the early hours of Friday morning.

Overall five people are now being detained and questioned by police - two that escaped from the burning car at Glasgow airport, two in Cheshire, and a fifth person was recently arrested in Liverpool.

One of the two men arrested at Glasgow Airport is currently in a critical condition after suffering severe burns, and is being treated at Royal Alexandra Hospital in Paisley.

In further developments, police have said that a controlled explosion took place outside the hospital on a car in connection to the Glasgow attack.

Scotland Yard has also reported that the two people arrested on the M6 in Cheshire overnight have been brought to London for questioning.

The UK has been moved up to its highest level of terror alert - 'critical', and a formal statement on the current situation will be made by the government in the House of Commons on Monday, as the government's emergency response unit, Cobra, held its fourth meeting in three days to tackle the increased threat.

Prime Minister Brown told the BBC that it was "clear that we are dealing, in general terms, with people who are associated with al-Qaeda".

He added: "It's obvious that we have a group of people - not just in this country, but round the world - who're prepared at any time to inflict what they want to be maximum damage on civilians, irrespective of the religion of these people who are killed or maimed are to be."

Brown urged the British public to remain "constantly vigilant", and also praised the "magnificent work" of police and security forces working against the extremists.

Security has also been tightened at airports throughout Britain and also across the US.

Lord Stevens, who is the new terrorism adviser in the Cabinet, has said the attacks signalled a "major escalation in the war being waged on us by Islamic terrorists".

The British Muslim Forum condemned the attempted attacks. Chair Khurshid Ahmed made a statement saying: "It is the duty of every British citizen to assist the police in safeguarding national security and ensuring the safety of all our citizens."