Major holes in the security of the air cargo industry, which could potentially threaten the lives of passengers, have been revealed in a BBC investigation.
An estimated 70% of air cargo is shipped on passenger airliners
It found cargo on passenger flights is being sent without being subjected to checks or X-rays.
The security gaps emerged following a court case over drug smugglers.
The findings come after tough new security measures were implemented in Britain following an alleged plot to blow up transatlantic airliners.
In August airports across Britain were thrown into chaos by tough new security measures applied because of a specific threat concerning hand luggage.
But after spending hours getting through security, many passengers would have been unaware that they were flying on planes whose holds contained tonnes of cargo - much of which had not been checked, inspected or X-rayed.
This happens every day as 70% of air cargo is shipped on passenger airliners.
Under a system called "known shipper" or "known consignor" companies which have been security vetted by government appointed agents can send parcels by air, which do not have to be subjected to any further security checks.
Unless a package from a known shipper arouses suspicion or is subject to a random search it is taken on trust that its contents are safe.
David Reynolds, flight safety and security officer of Balpa, the British Airline Pilots' Association, told Radio 4's World Tonight programme that "only a small proportion is X-rayed" and that if cargo comes from a known sender "the security checks (for passenger aircraft) will be exactly the same (as for pure cargo carrying aircraft)".
"The only additional checks will be if they are not from a known consignor where they will go through an X-ray or a physical check," he said.
John Goldsworthy, chairman of the European Express Association's Security Committee, said it would be impractical to scan everything.
He said as well as parcels, cargo might be "large indivisible loads", requiring a combination of security measures.
"Time, economy, the actual application of 100% security screening is not a practical option in that it would actually stop the operation," he said.
One reason the industry thinks security for air cargo is less of a problem is that it believes terrorists want to be certain of destroying passenger aircraft and that they could not be certain of doing that by shipping bombs as parcels.
However several large companies such as FedEx and UPS offer clients the chance to follow the progress of their parcels online.
This is a facility that Chris Yates, an expert on airline security for Jane's Transport, says could be exploited by terrorists.
"From these you can get a fair indication when that package is in the air, if you are looking to get a package into New York from Heathrow at a given time of day."
There is also evidence that in the air cargo industries other security measures can also be broken.
One industry source told the World Tonight that possibly as little as a half of air cargo leaving the UK is scanned - and in the US even less is checked.
Since 9/11 two people in the US have even managed to post themselves home by airmail - apparently it was cheaper than buying a ticket.
Captain Gary Boettcher, president of the US Coalition Of Airline Pilots Associations, says the "known shipper" system "is probably the weakest part of the cargo security today".
"There are approx 1.5 million known shippers in the US. There are thousands of freight forwarders. Anywhere down the line packages can be intercepted at these organisations," he said.
"Even reliable respectable organisations, you really don't know who is in the warehouse, who is tampering with packages, putting parcels together."
He added: "A package going from New York to Britain could have a bomb on it that is barometrically detonated so that when the plane descends down to British airspace the bomb goes off."
That may sound fanciful, but the BBC uncovered further evidence it could be all too easy.
In June this year a Nigerian student called Olumwaseum Adeyemi was sentenced to 11 years at Kingston Crown Court for importing cocaine into the UK.
Mr Adeyemi brought pounds of cocaine into Britain unchecked by air cargo, transported from the US by the Federal Express courier company. He did not have to pay the postage.
This was made possible because he managed to illegally buy the confidential Fed Ex account numbers of reputable and security cleared companies from a former employee.
An accomplice in the US was able to put the account numbers on drugs parcels which, as they appeared to have been sent by known shippers, arrived unchecked at Stansted Airport.
When police later contacted the companies whose accounts and security clearance had been so abused they discovered they had suspected nothing.
In a statement to the BBC FedEx said security was a top priority.
Mr Goldsworthy admitted the current vetting system would not have caught terrorists from 9/11 or the 7 July bombings because they did not have criminal convictions.
"That is correct, yes, but as I say there are quite stringent requirements within the air cargo industry for the vetting of personnel," he said.
Some pilots groups say the way to tackle the problem is to invest in new neutron scanning technology which they believe would allow all cargo to be scanned quickly and easily.
Mr Yates believes that the case for action is overwhelming.
"We have to get to the point of checking 100% of everything that goes on a passenger plane because as sure as night follows day one time somewhere down the line somebody will make an attempt to blow up an aircraft again."
It all adds up to a frightening prospect, a vetting system that would not stop potential terrorists working in sensitive areas and a package tracking system that could allow terrorists to target particular flights or even planes, bringing them and their passengers down when and where they wanted.
Dr Jim Swire, whose daughter died in the Lockerbie air disaster in 1988, said they were aware the known shipper system "was open to abuse".
He warned the emphasis on preventing suicide attacks could prevent the authorities from taking adequate action to deal with the very real threat of a bomb in the cargo hold.
"Of course what happened at Lockerbie was that there was an automated device in the hold of the aircraft and no terrorist on board. My message is you do not need a terrorist on board," he said.
FedEx told the BBC that it had "worked co-operatively and effectively with law enforcement and regulatory agencies around the world for a number of years to continuously strengthen our security program".
"Not only does FedEx comply with all country-specific aviation security and anti-terrorist regulations in both the UK and the US, but our security processes and procedures meet and often exceed regulatory requirements as well.
"The FedEx security systems consist of multiple layers of interlocking procedures and processes and contain a number of redundancies that comprise an excellent security system. We do not, however, discuss specific measures for obvious reasons."