The UK is bracing itself for a fresh wave of terrorist attacks following three attempted car bombings in London and Glasgow on Friday and Saturday.

Prime Minister Gordon Brown, who took office on Wednesday, said in a national televised address on Sunday that “it is clear that we are dealing with people who are associated with al Qaeda.”

Security authorities have long feared that the deadly car bombings which have become daily occurrences in Iraq would spread to European cities.

Late Saturday, the new Home Secretary, Jacqui Smith, raised the national terror threat to the highest level – ‘critical’ – which indicates that an attack is expected 'imminently'.

“This weekend’s bomb attacks signal a major escalation in the war being waged on us by Islamic terrorists,” said Lord Stevens, London’s former police chief and a terrorism adviser appointed earlier this week by Mr Brown. “Now al Qaeda has imported the tactics of Baghdad and Bali to the streets of the UK,” he wrote in an opinion piece published in Sunday’s News of the World.

British authorities said evidence indicated that the attack on Glasgow Airport, in which two men rammed a flaming Jeep into the entrance of the main terminal, had been a botched suicide mission.

One witness, Scott Leeson, said: “The car came speeding past. Then the driver swerved the car around so he could ram straight in to the door. He must have been trying to smash straight through.”

Some witnesses reported that the driver got jammed between concrete security bollards as he tried to plough into the terminal, which was bustling with families waiting to check in for flights on the first day of school holidays.

“They were obviously trying to get it further inside the airport as the wheels were spinning and smoke was coming from them,” said another witness, Lynsey McBean.

The driver then got out and poured gasoline over himself and the vehicle and lit them both, setting off a series of explosions.

“It was just a small fire at first,” said Robin Patterson. “Then there was an enormous explosion. The guy next to the car, his skin and clothes just fell off him.”

Engulfed in flames, the burning man threw punches at passengers and police and shouted “Allah! Allah!” as he tried to escape. He was restrained by police, who put out the flames with a fire extinguisher. Both the driver and his passenger were led away by police in handcuffs.

The driver was then taken to a local hospital to receive treatment. There, staff found on him a ‘suspect device’ believed to be a suicide belt, forcing the evacuation of the hospital. However, this proved to be a false alarm.

Five bystanders were also injured in the Glasgow Airport attack , none seriously.  On Sunday morning, the airport resumed flights, with passengers entering the terminal through a police cordon.

Glasgow police chief Willie Rae said Saturday’s airport attack and Friday’s attempted car bombings in London appear to have been connected. “There are clearly similarities and we can confirm that this is being treated as a terrorist incident,” he said.

Police discovered two Mercedes in central London in the early hours of Friday morning, each packed with gasoline-filled containers, gas cylinders, propane tanks and nails.

One car was parked outside the popular Tiger Tiger nightclub on Haymarket in London’s West End, a busy thoroughfare lined with clubs, theatres and restaurants and only a short walk from Piccadilly Circus. An ambulance driver treating one of the nightclub’s patrons for a head wound noticed vapors coming from the car and alerted police, who defused the explosives. The other car had been parked illegally on a nearby street and was towed away.

Police now believe that the bombers intended to use one explosion to draw panicking club patrons out on to the streets; then a second car bomb could have been detonated in the crowd to cause hundreds of deaths. This was the tactic used in the October 2002 Bali bombings, which killed 202 people.

Chris Driver-Williams, a retired British army major and now a terrorist bomb consultant to the US and British governments, told the Washington Post that the explosive firepower of the crude devices could have been “catastrophic”, especially since the valves on the gas cylinders were left open to create a fuel-air explosion.

This type of explosion uses oxygen to ignite a blast wave more powerful than many conventional explosives. However, these devices are difficult to ignite properly and could explain the flames coming from the Jeep even before its crash into the airport terminal in Glasgow.

Nevertheless, Mr Driver-Williams said that if such explosives were ignited properly, the result would be a “fireball the size of a house”.

The bombs assembled for the attempted London car bombings may reflect the increased restrictions on high-powered explosive materials in the UK.

“The danger here is that we are entering the era of the car bomb,” an unnamed  British intelligence source told The Independent. “In the past, al Qaeda-style terrorists have used high-explosive bombs aimed at symbolic, high-profile targets. But it’s easy to make a gas and nail car bomb without raising suspicion.”

Magnus Ranstorp, a Swedish terrorism expert has also said that the threat to the UK is the gravest in Europe because the UK is considered a close ally of the US and a major military partner in the invasion and occupation of Iraq.

It is not known yet whether the car bombers in London and Glasgow were assisted or funded by overseas terrorist organizations. Yet they are believed to be members of a loose network of terrorist cells that use the Internet to exchange information on the latest terror tactics.

Police have begun investigating the al Hesbah chat forum, which is frequently used by al Qaeda supporters. One message posted by frequent contributor Abu Osama al-Hazeen stated that “London shall be bombed” hours before the first car bomb was found in Haymarket. Police are trying to locate Mr al-Hazeen by following his ‘electronic footprint’.

Police are also using more conventional means to track down those involved, including forensic testing and security camera video footage. While the two Mercedes used were probably stolen, police have obtained a fairly clear CCTV image of one suspect who ran from the Mercedes in Haymarket at 1.30am on Friday. Investigators are now tracking his further movements using images taken by hundreds of cameras throughout central London. Police believe it is highly unlikely that those involved have tried to leave the country.

Five suspects have been arrested so far in connection with the incidents in Glasgow and London. In addition to the two men taken into custody at Glasgow Airport, police arrested two other suspects early Sunday on a major highway in Cheshire, northern England. A fifth suspect was arrested in Liverpool, where John Lennon Airport was closed overnight following the discovery of another suspicious vehicle. The airport has since reopened.

The UK’s new terror threat comes one week before the anniversary of the bombings which shook London on July 7, 2005. Just before 9 am that morning, three bombs exploded within fifty seconds of each other on Underground commuter trains; nearly an hour later, another bomb exploded on a double-decker bus. Fifty-two people were killed in the blasts, with more than 700 injured. It was later discovered that the bombings had been carried out by home-grown British Muslim extremists based in Birmingham.

British authorities have now ordered that security be tightened around the Wimbledon tennis tournament and Sunday’s concert honoring the late Princess Diana, which is expected to attract a crowd of 60,000 to London’s Wembley Stadium.