A health board is taking legal advice after a surgeon had to finish an operation by torchlight when maintenance workers switched off the power.
Two operating theatres at Edinburgh Royal Infirmary were affected by the incident.
An operation in one was postponed until power returned as it had not yet begun while in the other, staff finished the operation by torchlight.
It is understood that PFI provider Consort were to carry out planned maintenance work on the emergency power supply after surgery was finished but switched off the power too early, while surgeons were still at work.
NHS Lothian said they were consulting with lawyers.
Alan Boyter, executive director at NHS Lothian, said: "We have reached the point where we can no longer tolerate the repeated, serious and potentially life-threatening nature of these incidents at the Royal Infirmary of Edinburgh by our PFI provider Consort.
"We are currently consulting with our lawyers to discuss what options we have in relation to the contract and it would be inappropriate to comment further while that is ongoing.
"Patient safety is always our absolute priority and we will not allow that to continually be put in danger by a third party."
He added: "We are angry and frustrated with the performance of our PFI provider Consort."
He said that the health board have "detailed contingency measures" to ensure that patient safety is maintained. These include the availability of torches.
The health board would not comment on the nature of the operation finished by torchlight due to patient confidentiality.
The incident happened on 29 March.
Consort director Stephen Gordon said: "Consort has taken this incident very seriously and have undertaken a thorough investigation into this matter in conjunction with NHS Lothian to review the current operating procedures in place for works of this nature."
Labour health spokeswoman Jackie Baillie called for an independent inquiry.
She said: "It is a disgrace that in 21st century Scotland this has been allowed to happen - it is utterly unacceptable.
"The consequences could have been extremely serious indeed.
"NHS Lothian must, as a matter of urgency, ensure that the level of service provided by Consort improves dramatically. Patient safety must always be paramount."
Jim Eadie, SNP MSP for Edinburgh Southern, said he was "appalled" by the situation.
He said: "This is what happens when you put buildings and profit before patient care.
"NHS Lothian must conduct a full investigation into how this unacceptable situation was allowed to come about.
"Lessons must be learned to ensure that this never ever happens again."