Management at British Nuclear Fuels Ltd yesterday came in for strong criticism in an official report on falsification of fuel data at Sellafield.

The Nuclear Installations Inspectorate (NII) said that "systematic management failure" allowed individual workers to falsify quality assurance records.

Poor design of the Cumbria plant, the tedium of the job and the ease with which the computer dating logging system was manipulated were all blamed for the problem, which dates back to 1996.

But the report said that although data were falsified it would have no effect on the safety of fuel in a nuclear reactor.

The NII, part of the British Health and Safety Executive, published three reports into Sellafield, the largest nuclear facility in Britain.

It was already carrying out a mini-audit at the site but started a separate investigation when the falsification was discovered.

Mr Laurence Williams, chief inspector of nuclear installations, said: "The deficiencies found in the quality-checking process will have to be rectified, the management of the plant improved and operators either replaced or retrained to bring the safety culture in the plant up to the standard HSE requires for a nuclear installation."

The plant which manufactures uranium and plutonium mixed-oxide (MOX) fuel rods is now shut down, and Mr Williams said it would not be allowed to restart until the report's recommendations had been implemented.

Five process workers have been sacked over the falsification, and BNFL yesterday accepted its responsibilities following the report, although it is not expected to sack any executives.

An official from the NII visited Sellafield last year after managers reported that irregularities had been noticed by their quality-control team.

It was discovered that sampling of rods had not been carried out, and the records which showed it had been were copied from previous checks.

At the time of the discovery last year two shipments were already on their way to Japan.

Last week the Japanese government demanded that a shipment of MOX fuel be returned to Britain.

The report said that several process workers had not been following quality-control procedures.

"There is no doubt that data falsification took place, and MOX fuel assemblies have been produced and in some cases delivered to the customer with quality assurance documentation which included falsified data," it said.

The report makes 15 recommendations including the improvement of work stations and computer security.

BNFL is urged to identify the workers who deliberately falsified records and take "appropriate disciplinary action".

Employees who knew about the practice of falsifying records should be retrained and other staff made aware of the importance of following procedures.

The report also recommends that the roles of previous plant managers be reviewed as well as the suitability of the current management.

Another recommendation reads: "BNFL should ensure that any future management team members are aware of their responsibility to ensure the plant is operated to standards required of a nuclear establishment and that they are given sufficient time to spend on the shop floor talking to their staff."

The report adds that BNFL should "urgently consider" the implications of the incident for the Sellafield site and to report to the NII on how it intends to prevent a recurrence.

The 40-page report concludes: "The events which have been revealed in the course of this investigation could not have occurred had there been a proper safety culture within this plant.

"There can be no excuse for process workers not following procedures and deliberately falsifying records to avoid doing a tedious task. "These people need to be identified and disciplined.

"However, the management on the plant allowed this to happen and since it had been going on for over three years must share responsibility."

In a separate report the HSE said Sellafield lacked a high-quality safety management system.

Union officials described the reports as "devastating" and called for changes in the safety culture at Sellafield.

Mr Jack Dromey, national officer of the Transport and General Workers' Union, warned that a failure to act on the reports threatened the future of the state-owned company, which employs 20,000 workers.

The environmental campaign group Greenpeace said it was time for the British government to end nuclear reprocessing at Sellafield.

"The whole plutonium business is rotten to the core," said a spokesman, Mr Peter Roche.

BNFL said it fully accepted the reports and their recommendations and actions were already under way to improve the safety culture.

Mr Brian Watson, head of the Sellafield site, said: "Our response is that safety is and remains the company's top priority and of course we take these reports extremely seriously."

BNFL's newly-appointed chairman, Mr Hugh Collum, was conducting a "fundamental review" of management and would report back to the government within a couple of months, said Mr Watson.

He added: "There was no complacency within the management about what needs to be done. We will take on board all of the recommendations. We have been investigating procedures very thoroughly already and we have acted on a number of issues."

Mr Watson said he could not comment on whether any executives would have to resign or be sacked over the problem but added he did not anticipate any further dismissals of process workers.

He said that the episode had been "damaging" for BNFL.

The Energy Minister, Ms Helen Liddle, said she was "incensed" at how things had gone so badly wrong at Sellafield and she was expecting a thorough management review to be carried out.