An intensely radioactive beam leaked out from a protective flask on a three-hour journey across northern England after engineers forgot to put the lid on.

Leeds Crown Court heard how it only "pure good fortune" that no-one was dangerously contaminated when a plug was left off a 2.5 tonne container carrying radioactive material on a lorry.

The flask belonging to AEA Technology was being used to transport a piece of decommissioned cancer treatment equipment 130 miles from Cookridge Hospital, in Leeds, across the Pennines to the Windscale site on the Sellafield complex in Cumbria on March 11, 2002.

A judge was told how the container was "found to be emitting a narrow collimated beam of radiation, of a very high dose rate, vertically down from that package base".

He heard how the leak was present both at the hospital and Windscale, as well as throughout the three hour-long road journey between the two.

Mark Harris, prosecuting for the Health and Safety Executive, said: "Through pure good fortune no-one involved in the removal, containment and transfer of the source may have been directly exposed to the radiation beam.

"The risk of such exposure was undoubtedly present - at Cookridge, during the journey and at Windscale. That occurred because a shield plug - an integral part of the approved packaging in which the source was required to be carried - had been omitted."

Mr Harris went on: "We say the incident was serious. It exposed employees of the company and their sub-contractors to unnecessary and potentially high radiation risks due to poor radiation protection practices.

"The radiation dose rates measured at Windscale were in the order of 100 to 1,000 times above what would normally be considered a very high dose rate, and measurement was beyond the capabilities of normal hand-held monitoring equipment."

Mr Harris told the court it was fortunate the beam had been pointing vertically down. He said that if an accident had caused it to emit horizontally the beam would have emitted dangerous radiation for 980ft.

The firm AEA Technology - which is a privatised arm of the UK Atomic Energy Authority - has admitted a series of breaches of Health and Safety regulations, the Ionising Radiations Regulations and the Radioactive Material (Road Transport) Regulations.

It was due to be sentenced at Leeds Crown Court today by Judge Norman Jones who decided he needed more time to read papers and postponed setting the level of the fine until Monday.

The HSE has already asked for costs of £151,323.

John Hand QC, defending, told the court the company lost £1 million following the incident as it reorganised the subsidiary involved, which it has now sold off. Mr Hand admitted employees of the firm had been "relaxed and somewhat cavalier" at the hospital and had even ticked forms to say they had completed tasks which they had not.

Judge Jones said: "We have to remember here we’re dealing with the movement in public areas and, long distance movement, with very, very dangerous materials and therefore the greatest of care is demanded of those engaged in that movement."

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