The two men, aged 25 and 28, were arrested overnight in the wake of Saturday's attack in which a blazing car loaded with gas canisters ploughed into the door of the main terminal of Scotland's busiest air hub.

That attack followed two failed car bombings in London on Friday, and triggered the government to raise the national security threat level to its highest, meaning another attack is expected "imminently."

"I'm very encouraged by the progress" made in the investigation, said Home Secretary Jacqui Smith, who was to update lawmakers on the probe during the day.

Amid fears of a new al-Qaeda-style attack, the government also called a new meeting of its so-called COBRA crisis cell, the fifth in four days, to discuss the latest developments.

The suspects already in custody include the two men who rammed a Jeep Cherokee into Glasgow airport, along with a Jordanian surgeon named by officials in Amman as Mohammed Jamil Abdelkader Asha, and his wife.

Asha is of Palestinian origin and carries a Jordanian passport, the officials said. He obtained his medical degree in Jordan.

London's Metropolitan Police, which is coordinating the anti-terrorism operation, refused to comment on BBC reports that all those arrested were from the Middle East.

Before the new arrests Monday, press reports indicated that police were looking for at least one more person suspected to be linked to an al-Qaeda cell intending to launch attacks in Britain.

Police say there are "ever clearer" links between the Glasgow attack and the failed bombings in London, in which two Mercedes cars laden with gas canisters and nails were abandoned in the capital's entertainment district.

The seven arrests have come in rapid succession.

They began on Saturday when police detained the two men who drove the car into Glasgow airport. One of them remains in critical condition in hospital with severe burns.

Hours later anti-terror officers arrested a 26-year-old man and a 27-year-old woman, believed to be Asha and his wife, on a motorway in northwest England. They are now being questioned in London.

The fifth suspect, a 26-year-old man, was arrested at Liverpool's Lime Street Station on Saturday night.

The two latest arrests were made following "intensive police operations" west of Glasgow overnight, police said, indicating that the suspects are not believed to be of Scottish origin.

"This continues to be a fast-moving investigation and I am grateful to the public for their perseverance and support during these difficult times," said Assistant Chief Constable John Malcolm of Strathclyde Police.

Prime Minister Gordon Brown, who succeeded Tony Blair last Wednesday, vowed Sunday that Britain "will not yield, we will not be intimidated" by terrorism -- and hinted that al-Qaeda-linked militants may well be behind the attacks.

"It is clear that we are dealing in general terms with people who are associated with al-Qaeda in a number of incidents that have happened across the world," he told the BBC.

Several British newspapers published gruesome photos of one of the suspected Glasgow attackers, writhing in pain on the ground outside the airport with severe burns.

The attacks prompted the government to raise the security alert level to "critical" for the first time since August, and about a week before the second anniversary of the July 2005 London bombings that left 52 commuters dead.

"Critical" is the highest on a five-level scale, and means another attack could be imminent.

The British terror alert also triggered alarm abroad.

The United States beefed up security at airports following the attempted attacks in Britain, while Homeland Security Secretary Michael Chertoff said Sunday that extra US air marshals were being deployed on flights to Britain.

He said the US administration took seriously threats of terrorism emanating from Europe, including Britain.

"It's one of the reasons we've been very focused on increasing our security for people incoming from Europe," he told US television. "And that's something we're going to be looking at for the rest of the summer."

US President George W. Bush hailed Brown's "very strong response" to the attempted attacks.

In Paris, French Interior Minister Michele Alliot-Marie called a meeting with security chiefs to discuss events in Britain.