The hospital was built under the PFI initiative

Stand-by generators broke down and only batteries maintained supplies to critical parts of the hospital, like intensive care after the main power supply was lost.

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

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

Lothian NHS Board is demanding urgent talks with the private company that runs the hospital, which opened in January.

I want a report from NHS Lothian about this incident as a matter of urgency to see what lessons need to be learned Malcolm Chisholm

Board chairman Brian Cavanagh described the incident as "intolerable" and called for an immediate inquiry.

He said: "We have a duty of care to our patients and staff and anything which breaches that is totally unacceptable.

"In view of the seriousness of this situation we are demanding an immediate meeting with Consort Healthcare, our PFI partners who are responsible for the fabric and the maintenance of the hospital.

"We want absolute assurances that this problem with the generators will be sorted out once and for all."

'Generator fault'

Minister for Health Malcolm Chisholm said he was extremely concerned about the incident

"Patient safety was obviously paramount and I commend all the efforts of the staff in caring for their patients in what must have been a very stressful period.

He added: "This was a very serious failure. I want a report from NHS Lothian about this incident as a matter of urgency to see what lessons need to be learned."

A spokeswoman for Consort Health Care, which is responsible for non-clinical services at the hospital, acknowledged there'd 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.

Last month, about 40 members of staff walked out in protest at high temperatures in the building during the heatwave.