Japan has formally asked Britain to take back a controversial consignment of plutonium-based nuclear fuel.

The request marks the failure of a UK delegation's efforts to find consensus on the issue.

British Nuclear Fuels Limited (BNFL) has admitted its employees saved time by by-passing quality control checks on mixed plutonium-uranium oxide (Mox) fuel at the Sellafield plant in the north of England.

Some test data were falsified on the size of fuel pellets in a shipment of Mox sent to Kansai Electric Power in Japan.

Anna Walker, director-general for energy at Britain's Department of Trade and Industry, said Japan's Ministry of International Trade and Industry had requested that the fuel be sent back.

"But it is a complex matter which raises contractual issues between companies as well as between governments," she said.

"We will look at our views on this and come back to the Japanese Government on it. Both sides have agreed to further discussions."

The Japanese Government banned imports of Mox fuel from Britain in December, after the false figures came to light.

BNFL has repeatedly said the problem is one of "quality assurance" rather than one of safety, but Japanese environmental groups have said they are unconvinced of the fuel's safety.

The UK Government's Nuclear Installations Inspectorate is due to publish a report on the falsification incident next week which will recommend "quite sweeping and radical measures" to ensure there is no repeat, Ms Walker said.

BNFL, which has already sacked three workers over the incident, is carrying out its own investigation.

Embarrassment

The incident has been highly embarrassing for the British authorities.

Earlier in the week an official apology was delivered to Hirofumi Kawano, director-general of the agency of natural resources and energy at the Ministry of International Trade and Industry.

If Japan continues with its ban on using reprocessed fuel, the brand-new Sellafield reprocessing plant may be forced out of business even before it goes into full operation.

Japan is the largest potential customer for the new plant, which has yet to receive UK Government approval.

Japan's request for the fuel to be returned suggests that an end to its ban is still some way off - a blow to BNFL, which counts on Japan for some 60% of its operating profits.

The shipment was a rare transfer of "first-use" nuclear material considered easily convertible into nuclear weapons and was dogged by controversy and protests.

At the insistence of the United States, the consignments had to be shipped in specially-adapted and heavily-armed vessels to prevent the fuel falling into the wrong hands.