British police arrested two more suspects in a hunt for members of a suspected al Qaeda cell which rammed a jeep packed with fuel into an airport and left two car bombs in London, police said yesterday.

Cars and other vehicles were banned from directly approaching airports and security measures were stepped up across the country as authorities kept the threat level at ?critical?, meaning the possibility of an attack is ?imminent?.

Government ministers said good progress was being made in the investigation and police sources said it was highly likely more suspects would be detained. The latest two arrests, made on Sunday night, took the number of people detained to seven.

Police said the two men, aged 25 and 28, were are not believed to be of Scottish origin and were detained as a result of an ?intensive police operation? in the Paisley area, which is west of Glasgow, Scotland?s biggest city.

?This continues to be a fast-moving investigation,? said Assistant Chief Constable John Malcolm of Strathcylde police in Scotland. ?I would continue to urge people to be vigilant.?

A British security source said it was premature to say whether all those arrested were foreigners. ?That?s still an area that?s being looked at. That?s far from certain, as to what nationalities the people are.?

The attacks are a stiff test for Prime Minister Gordon Brown, who last week replaced Tony Blair. In 2005, Britain was the first country in Western Europe to be hit by Islamist suicide bombers and since then several plots have been foiled. Brown has said he believes those behind the botched attacks were associated with militant group al Qaeda.

Blair was known for an aggressive stance on security and a foreign policy which strongly supported the United States in Afghanistan and Iraq. The home-grown bombers who struck London transport two years ago, killing 52 commuters, said in videos they were punishing Britain for Blair?s policies.

Home Secretary (interior minister) Jacqui Smith said Britain was facing a ?serious and sustained threat of terrorism? and urged the public to remain on alert.

British newspapers said two of those who were arrested were doctors ? one an Iranian who worked at North Staffordshire Hospital in central England. A spokeswoman at the hospital declined to comment.

On Saturday, police arrested the passenger and badly-burned driver of a Jeep Cherokee who had rammed the vehicle into the entrance of Glasgow?s airport, causing a huge fireball.

The attack came 36 hours after police in London defused two Mercedes car bombs packed with fuel canisters, propane tanks and nails, parked in the heart of the capital?s bustling theatre and nightclub district.

The other arrests included a 26-year-old man and a 27-year-old woman seized on a motorway in northern England on Saturday, and a 26-year-old man in Liverpool on Sunday. A police spokesman said two of the suspects had been taken to London for questioning yesterday.

Police and ministers said protective security measures would be stepped up across Britain, particularly at transport hubs.