The move comes after the operators, Rolls-Royce, confirmed it had been warned about serious safety flaws at the factory in Raynesway, Derby.

The BBC's Linda Duffin: "Rolls Royce have since changed their procedures "

The Sunday Times says the company is processing fuel even more volatile than the material that sparked a nuclear scare in Japan last month.

Highly enriched uranium fuel is treated in a special wing of Rolls-Royce's plant.

It is delivered by the Ministry of Defence (MoD) and, after treatment, is used in Royal Navy nuclear submarines.

Radioactive clouds

The Sunday Times says it has seen leaked Rolls-Royce documents that admit there is a risk of a "criticality accident" similar to that which caused the disaster at the Tokaimura plant in Japan last month.

In that incident radioactive clouds drifted over several inhabited areas after a nuclear fission chain reaction.

The Derby plant makes engines for nuclear subs

Three workers were seriously injured and a further 36 contaminated after the radiation leak.

The fuel involved was 20% uranium 235 but at Derby it is reportedly 90% uranium 235.

A Rolls-Royce spokesman said the Nuclear Installations Inspectorate (NII), part of the Health and Safety Executive, had found flaws in its safety procedures during an exercise in March.

He said: "We have now clarified the position, changed our procedures and practised them three times."

He added: "Last week we had an emergency exercise involving the emergency services which was observed by the NII.

"Afterwards, the NII said it was now satisfied with our procedures."

Defence Secretary Geoff Hoon said the plant should not be closed.

"Those concerns have been addressed by the Nuclear Installations Inspectorate and they've given the plant an entirely clean bill of health.

"Those concerns have been thoroughly investigated."

The factory is in the heart of Margaret Beckett's Derby constituency

A Health and Safety Executive spokeswoman said there had been safety concerns at the plant.

She said: "There was an emergency exercise in March following which we asked for some improvements and a repeat of the exercise, which took place on Wednesday.

"The NII was satisfied."

The Sunday Times says the city's 223,000 residents were unaware of the factory's clandestine role because it was "classified".

The factory's main role is as a manufacturer of engines for Trident nuclear submarines.

As such it is highly sensitive, and lorries are often escorted into and out of the plant by armed guards.

Derby also has no emergency plan in case of a nuclear leak, reports the paper.

'Right to know'

Dave Knight, chairman of the Campaign for Nuclear Disarmament, called for the Raynesway plant to be closed down immediately.

He said: "Urgent answers are needed to some very serious questions.

William Peden from Campaign for Nuclear Disarmament: "It's an unsuitable operation to have in the centre of Derby"

"Why is there no emergency plan? Are there any containment procedures for a critical accident? The people of Derby have the right to know.

"As Tokaimura showed, accidents do happen and when they do the effects are long lasting if not deadly for the local populace."

The city's MP, Margaret Beckett, a former Cabinet Minister and currently Leader of the House of Commons, is expected to raise the matter with the new Defence Secretary Geoff Hoon.