Perimeter security is notoriously difficult to enforce, they say, particularly at major international airports such as Heathrow where it extends for several miles.
Security experts said that there were few physical measures the airport could take to prevent mortar or missile attacks on the airport, or on aircraft taking off or landing. Good intelligence was the best counter-measure, one expert said last night.
Regular police and private security patrols monitor perimeter fencing and access points which allow vehicle entry to the airport apron.
Heathrow now handles more than 47 million passengers a year who use more than 80 airlines.
Security was tightened at all major airports after the Lockerbie disaster and terrorist gun and bomb attacks at Rome and Vienna airports in 1985. There is close liaison between Scotland Yard and airport and airline security officials.
More security staff are on duty at airports and baggage screening procedures are now far more thorough.
For several years police on duty at Heathrow have been armed and at times of heightened tension - such as during the Gulf war - the Army has patrolled the airport's perimeter fences.
'We have comprehensive emergency orders which set out procedures to follow and these are the procedures, for example, that we have followed tonight,' said a Heathrow spokeswoman.
There are also continuous security exercises involving airport staff, police and the other emergency services.
Once a year there is a major exercise which Heathrow has to carry out to satisfy the Civil Aviation Authority. This exercise usually involves the scenario of a major crash or terrorist attack on the airport.Reuse content