Anti-nuclear campaigners have threatened legal action to stop a cargo of plutonium which is expected to leave Japan this week bound for Cumbria.

Two ships, the Pacific Pintail and Pacific Teal - carrying cargo large enough to make 50 nuclear bombs - are due to arrive in the Irish Sea in August.

The load will be taken to the controversial multi-million pound mixed oxide (Mox) plant at Sellafield.

The fuel was taken to Japan more than two years ago, but the Japanese abandoned plans to use it when it emerged that quality control checks had been forged.

These are among the safest ships that sail the oceans

BNFL spokesman

Those opposed to its transportation say the fuel - a blend of uranium and plutonium oxides - could be converted into nuclear weapons.

A Greenpeace spokesman said: "We believe this transport is unlawful.

"We have written to British Nuclear Fuels (BNFL) and the Environment Agency to tell them so, and we are prepared to take legal action to prevent it."

Greenpeace believes the growing threat of international terrorism since 11 September makes the convoy vulnerable to attack.

A spokesman for BNFL refused to comment on security but said the ships "are among the safest that sail the oceans."

The vessels left Britain five weeks ago and it is thought they will enter the North-West Pacific Basin this week.

Security matters

Britain was ordered to take back the contentious cargo after details of the forgery emerged.

The BNFL hopes the return of the rods will draw a line under the affair.

A BNFL spokesman said: "We gave an undertaking to our customers that we would bring the fuel back in 2002 and that is what we are going to do.

"For obvious reasons we do not discuss cargo routes and I obviously cannot talk about detailed matters of security.

"These are among the safest ships that sail the oceans.

"They have a number of safety features and are double hulled and reinforced to withstand collisions."