TWO litres of nitroglycerin were carried on a passenger flight from the UK's busiest airport in one of almost 900 dangerous goods incidents reported to the UK's aviation watchdog in the last five years.

The flammable liquid, the key ingredient in dynamite, is one of the world's most powerful and unstable explosives. It is particularly dangerous to transport as heat or physical shock, such as turbulence, can cause it to combust.

However, The Herald has learned that on February 22, 2013, a bag containing two litres of the chemical was found during post-flight luggage screening at Zagreb Airport. The goods had been loaded at London Heathrow. Croatian police were called and an investigation launched.

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It is unclear which airline was involved, or the concentration of the nitroglycerin.

Chemists say it was an "extremely large amount", which - if pure - could almost certainly have blown up the aircraft, suggesting the chemical had probably been diluted in ethanol and may have been transported for medical purposes. It can also be used to treat angina.

However, Jamie Gallagher, chemist and science spokesman at Glasgow University, said it should never have been on a plane unless authorities were sure it was safe.

He said: "What you can say from this is that it wasn't being transported safely or correctly. Someone has broken regulations and potentially hazardous goods have ended up on board which may have caused a disaster on the aircraft."

The incident, reported to the Civil Aviation Authority (CAA) by the aircraft operator, is among hundreds of alarming security breaches that have occurred on flights to and from the UK since 2009, and the numbers are on the rise.

Figures obtained under freedom of information reveal there were 65 dangerous goods incidents reported to the CAA in 2009. By 2013 this had soared to 278 reported incidents, with 179 by July 17 this year.

The CAA said the increase was down to better training and reporting, increased passenger journeys, and a clampdown on errors involving electric wheelchairs and other mobility devices.

Only British airline and freight operators are required to report safety breaches to the CAA, with foreign carriers such as Qantas, Emirates or Lufthansa reporting dangerous goods incidents to their own national authority.

Among the 894 safety lapses was the transportation of 1023kg of "undeclared radioactive material" from Heathrow to China without paperwork; fuel-soaked blankets transported in the hold of a Boeing 777 passenger plane from Abuja, Nigeria to Heathrow; "five boxes of undeclared dangerous goods" discovered in the hold of a Boeing 777 on arrival at Heathrow from Ghana; "three pieces of dangerous goods loaded and travelled without pilot's knowledge" on a Boeing 777 flight from Heathrow to Israel; and eight sporting weapons and two rounds of live ammunition flown on an Airbus 319 passenger plane from France to UK, with airport ground staff only realising the weapons were on board once the plane was in the sky.

Dangerous goods can include paint, lighters, aerosols, and even party poppers, as well as poison, acid, radioactive or infectious substances, and explosives.

A spokesman for pilots' union Balpa said: "Each of these breaches is a serious matter which we hope the authorities have investigated thoroughly and taken steps to prevent happening again."

A spokesman for the CAA, said the body worked very closely with shippers, freight forwarders, airlines and their handling agents to ensure dangerous goods were carried in a safe way.

A Department for Transport spokesman would not comment on individual cases. He said: "The independent CAA is the body responsible for checking that the appropriate measures are in place to ensure compliance."