The inquiry follows an incident last week when a digger cut through electrical cables, paralysing the plant after the emergency supply also failed, prompting concerns about the control of the plant.

The investigation is expected to last four weeks and cost the taxpayer at least �200,000.

Intensive investigations will begin next month with 10 inspectors drawn from the HSE and the Scottish Environmental Protection Agency (Sepa) with broad terms of reference.

Experts believe it will be the largest investigation to be mounted since the inquiry into how a radioactive slick was washed ashore at Sellafield in Cumbria, northern England, more than a decade ago.

The review is to be more thorough than an inspection ordered earlier this week by the Nuclear Installations Inspectorate, an arm of the HSE, into the electrical workings of the plant.

Earlier this month, Sepa announced plans to curb radiation pollution at Dounreay, after plant officials admitted that they had miscalculated radiation levels.

In April, Dounreay received a consignment of uranium which was imported for processing from the former Soviet republic of Georgia, in a highly controversial move.

Scottish Secretary Donald Dewar welcomed the investigation

The British Government said it had accepted the uranium to stop it from falling into terrorists' hands, but the decision was criticised by Labour backbenchers, the Scottish National Party and environmentalists.

Dounreay has now been ordered to cease all reprocessing until the inspection is complete, which will delay the processing of the Georgian fuel.

A joint audit of management of nuclear safety by the HSE and Sepa, planned for later this year, was widened and brought forward after the incident.

HSE's Director General, Jenny Bacon, said: "HSE and Sepa are both determined to ensure that safety at Dounreay is properly managed and are bringing forward and extending this safety audit in view of recent events and the continuing level of concern that has been expressed about safety at the Dounreay site.

"We will produce a report of the audit and would hope to be able publish it later this summer."

Dr George Hunter, head of policy co-ordination for radioactive substances, said: "There are obviously concerns about a few incidents. But the most serious one is a loss of power in terms of nuclear safety.

"The terms of reference are pretty wide and it is on a par with the investigation into Sellafield. Primarily the audit is being done because of the power failure."

Environmental groups have been the most vocal in calling for change at Dounreay

The terms of reference include the management of reprocessing operations, radioactive waste and decommissioning; the arrangements for dealing with incidents; and the interface between Dounreay and the UK Atomic Energy Authority's corporate management.

Lang Banks, of Friends of the Earth (Scotland), said the review was more than FoE had imagined but called for a cessation of work on the site until the audit had been carried out.

Mike Townsley, of Greenpeace International, said the situation had become "farcical" and called for the plant to close.

"If the government is genuinely interested in creating jobs in the north of Scotland, there are cheaper and less destructive ways of doing it."