The national terror threat level has been raised to critical, its highest level, Home Secretary Jacqui Smith has said.
The level means an attack is expected "imminently". It has been raised as a response to the events of the last 48 hours, which have seen a blazing car smash into Glasgow airport and the planting of two car bombs which failed to detonate in the heart of London.
The Joint Terrorism Analysis Centre (JTAC) raised the level and appropriate security measures have been put in place, Ms Smith said.
Ms Smith had earlier attended a third meeting of the emergency Cobra committee along with the Prime Minister at Downing Street.
In a televised statement in 10 Downing Street following the meeting, Gordon Brown urged the British people to be "vigilant" and to support the police and security services in their counter-terror measures.
Mr Brown, who chaired two meetings of the emergency contingencies committee on Sunday, said it was right to raise the alert to its highest level of "critical".
He said: "Let me first of all thank the police, the security services and all the emergency services for the dedicated professionalism that has been shown in responding to the incidents yesterday in London and now today the attack on Glasgow Airport.
"The first duty of the Government is the security and safety of all the British people, so it is right to raise the level of security at airports and in crowded places in the light of the heightened threat.
"I want all British people to be vigilant and I want them to support the police and all the authorities in the difficult decisions that they have to make.
"I know that the British people will stand together, united, resolute and strong."
