The UK is on its highest level of alert - critical - meaning an attack could be imminent.
It follows yesterday's car-bombing of Glasgow Airport, coming just one day after two devices were found in London.
The two incidents are being linked by police.
Two men were arrested immediately after the attack in Glasgow while two other people were detained by police in a follow up operation.
Armed officers are now patrolling many of Britain's airports and access is being restricted.
The British Prime Minister Gordon Brown has chaired a meeting of the emergency Cobra committee and he said: "The first duty of a government is the security and safety of all British people so it is right to raise the levels of security at airports and private places in the light of the heightened threat."