The hospital was built under the PFI initiative

The hospital was left without electricity for more than an hour after power failed and stand-by generators broke down.

Only batteries maintained supplies to critical parts of the hospital, such as intensive care, after the main supply was lost.

According to unions, staff and patients were locked in wards because electric doors would not open.

Staff also had to use handheld breathing apparatus on some patients because the power failure had rendered machines useless.

Health Minister Malcolm Chisholm said that lessons had to be learned to ensure that the situation does not recur.

"I am shocked and extremely concerned over what happened," he said.

"This simply should never happen and we must ensure that it never happens again."

The failure had been initially blamed on a National Grid failure but Scottish Power later revealed there had been no interruption in the public supply to the infirmary.

Clinicians, staff, visitors and patients should be able to come to a hospital where the place is secure Brian Cavanagh Lothian NHS Board

It is now thought that a fault in the hospital's own "fail-safe" system triggered the problems.

David Forbes, Unison spokesman, said the situation was "nerve-racking" for those inside the hospital.

"The situation saw people locked in wards because the electronic doors had failed.

"There was hand bagging - where staff had to keep patients alive using handheld breathing equipment.

"It was a very nervous and nerve-racking situation."

Brian Cavanagh, chairman of Lothian NHS Board, branded the incident "intolerable".

He added: "Clinicians, staff, visitors and patients should be able to come to a hospital where the place is secure.

"It is intolerable that the public and staff should be subjected to an inability of the contractor to deliver on its contractual obligations."

Mr Cavanagh has called for an immediate inquiry.

A spokeswoman for Consort Health Care, which is responsible for non-clinical services at the hospital, acknowledged there had been a generator fault which, she said, was being investigated so that there would be no repetition of the incident.

The hospital was built and is run under the Private Finance Initiative.