GLASGOW, Scotland — A Jeep Cherokee trailing a cascade of flames rammed into Glasgow airport on Saturday, shattering glass doors just yards from passengers at the check-in counters. Police said they believed the attack was linked to two car bombs found in London the day before.

Britain raised its terrorism alert to “critical” — the highest possible level — and the Bush administration announced plans to increase security at airports and on mass transit.

Five bystanders were wounded, though none seriously, police said.

One of the men in the car was in critical condition at a hospital with severe burns, while the other was in police custody, said Scottish Police Chief Constable Willie Rae. He said a “suspect device” was found on the man at the hospital and it was taken to a safe location where it was being investigated.

Rae would not say whether the device was a suicide belt. British security officials said evidence pointed toward the Glasgow attack being a suicide mission.

“I can confirm that we believe the incident at Glasgow airport is linked to the events in London yesterday,” Rae said. “There are clearly similarities and we can confirm that this is being treated as a terrorist incident.”

Police foiled the plot Friday after two cars were found in central London packed with explosives — one outside a nightclub near Piccadilly Circus and another parked nearby.

A British government security official said the methods used in the airport attack and Friday’s thwarted plots were similar, with all three vehicles carrying large quantities of flammable materials.

The official spoke on condition of anonymity because of the sensitivity of the information.

Police and MI5 had no specific intelligence warning of a plan to attack Scotland, but they have monitored a host of suspected terrorists and plots there, he said. It was not yet clear whether there was an international element to the planning or funding of the attacks, the official said.

The new terrorist threat presents Prime Minister Gordon Brown, a Scot who took office on Wednesday, with an enormous challenge and comes at a time of already heightened vigilance one week before the anniversary of the July 7 London transit attacks, which killed 52 people.

“I know that the British people will stand together, united, resolute and strong,” Brown said Saturday in a televised statement.

President Bush was being kept informed of the situation, the White House said. “We’re in contact with British authorities on the matter,” said Gordon Johndroe, a spokesman for the National Security Council, in Washington.

The green Jeep barreled toward Glasgow’s main airport terminal shortly after 3 p.m. Leeson said bollards — security posts outside the entrance — stopped the driver from driving into the bustling terminal, but the nose of the vehicle smashed the glass doors.

“If he’d got through, he’d have killed hundreds, obviously,” he said.