BNFL has tried to play down fears that radioactive gas regularly put into the air from Sellafield is a cancer-causing danger to the general public.

The discharge of Krypton 85, a by-product of spent nuclear fuel reprocessing, has been an emotive health issue for several years.

BNFL has had to defended itself against claims that Sellafield releases a "killer gas" and has done nothing to stop it.

The allegations have re-surfaced in The Guardian.

BNFL said Krypton 85 is discharged under authorisation and gives an annual radiation dose, to those most exposed, which is equivalent to spending about an hour in Cornwall.

"Independent scientific research has concluded that the release of Krypton 85 does not have a detrimental effect upon the atmosphere. BNFL has actively researched Krypton 85 removal technologies for the last five years. To date, there is no system available or which could be developed capable of safely removing and storing Kr 85 on an industrial process sale."

The company stuck by the statement despite the leak of an internal BNFL document sent from one of its Risley-based public affairs officers (Rupert Wilcox-Baker) to Matthew Simon, who was Thorp manager at the time.

Against a background of Japan proposing to build its own reprocessing plant, the document allegedly read: "What we sell or give Japanese Fuels on Kr85 is a commercial decision. What we must do is persuade them not to fit Kr85 removal equipment as this is damaging to our own position."

Although BNFL refused to comment on a leaked document, the purported content is believed to be accurate.

The letter also allegedly said that Japan's bid for consent to operate its own reprocessing plant must include "safety and environmental information that to capture krypton is less acceptable than releasing it to the environment."

There was an angry reaction from Cumbrians Opposed to a Radioactive Environment (CORE).

Its spokesman, Martin Forwood, claimed: "The leaked document shows BNFL conniving to prevent a competitor installing gas retention equipment that it claims does not exist. BNFL should be made to come clean about whether the technology exists because this document throws doubts on its claims.

"This disregard for the health of innocent people all over the world will not go down well in Japan, where the company is trying to regain contracts it lost during the MOX data falsification."

The National Radiological Protection Board says Sellafield's Kr releases could cause two fatal skin cancers a year and 100 other cancers. BNFL maintains that this is theoretical and calculated on a collective radiation dose to huge numbers.