Surgeries have been cancelled following a black out at Perth Royal Infirmary.
The hospital was plunged into darkness when its main supply cut out just after 10pm on Saturday.
Emergency and GP services were temporarily transferred to Ninewells, while engineers battled to restore power.
They have now discovered a fault in the hospital’s main system, which will require specialist repair work.
Staff have been relying on back-up generators since the power failure.
NHS Tayside, which revealed that several operations had been cancelled as a precaution, said it hoped to have power fully restored by Tuesday evening.
Workers have been praised for their professionalism during the outage, which lasted for about 45 minutes.
A spokeswoman said: “A team of engineers from NHS Tayside and SSE were on site immediately and managed to restore power to affected areas of the hospital, however the disruption has identified an issue with part of our electrical system which requires a specialist repair.
“There is now power to all areas of the site, however the clinical team decided to postpone a number of elective procedures at PRI on Monday and Tuesday, until the necessary repairs can be carried out.”
Operational Medical Director Professor Peter Stonebridge added: “We would like to apologise to any patients affected, however patient safety is always our first priority and we hope people would understand why our clinicians have had to make this decision.
“We have contacted patients directly and are working to reschedule their procedures as soon as possible.”
He said: “The repair was scheduled to take place during the evening to minimise impact across the site and we expect all services to be running as normal from Wednesday.
“We would wish to thank our staff for their professionalism and continuing excellent work to minimise impact on services for our patients during this period.”
