Increased security measures imposed on passengers at Heathrow Airport could create new targets for terrorism, an expert has warned MPs.
The increased security levels lead to disruption in August
Security was stepped up after the foiling of an alleged plot to blow up transatlantic airlines in August.
But Professor Alan Hatcher, principal of the International School for Security and Explosives Education, said passengers were still vulnerable.
People queuing to have baggage screened in terminals could be a risk, he said.
Prof Hatcher was giving evidence to the Commons transport committee, which is looking at the lessons that can be learned from the 10 August security scare.
"We have lines of people in terminals, 200 or 300 people in a queue," he said.
"Your bag is not searched when you go in. You are allowed to take 23 kilograms of baggage with you - 23kg of ammonium nitrate mix would be very sufficient to make a good impact.
"Terminal three is especially vulnerable."
His warning was echoed by Michael Todd, the chief constable of Greater Manchester Police.
He said security measures were concentrated on the controlled zone after passengers had been through security, even though the area of vulnerability at airports was external.
"You won't bring down an aircraft, but actually you could disrupt an airport very, very easily through an attack outside or certainly you can attack confidence."
Mr Todd, who was speaking on behalf of the Association of Chief Police Officers, called for a 50p airport tax for every passenger to fund airport security.
But Craig Bradbrook, director of security and facilitation at the Airports Council International, argued this would not be fair.
"Terrorism is not targeting airports, it's targeting governments and we are just the means through which the terrorists take their battle forward," he said.
Car park risk
It was not sustainable in the long term to "burden the industry with the costs" of what involved a threat to national security, he said.
Prof Hatcher also said he believed it would be possible for a terrorist to take a weapon off an armed police officer in the secure area of an airport.
"Terrorists are extremely well trained. These people are not stupid. It they think there's a real chance to take out a bobby who is giving advice to a member of the public, yes, his weapon could be taken," he said.
He and Mr Todd agreed that short stay car parks close to airports were a potential terrorist threat.
Thousands of flights were cancelled at Heathrow and BAA's other UK airports after the alleged plot was foiled on 10 August.
A total ban on hand luggage was initially brought in but after a few days passengers were allowed to carry on a laptop computer-sized bag.