A health board is taking advice from their lawyers after surgeons had to carry out an operation by torchlight when the power was cut off.
Staff from the private company which runs Edinburgh Royal Infirmary started maintenance work too early and switched off the lights while procedures were still being carried out.
Medical staff had to use torches to light the patient and emergency procedures kept monitoring equipment running. Another patient who was due to have surgery had their procedure cancelled.
On Thursday, NHS Lothian said it was consulting its lawyers to discuss what options its has, saying it "can no longer tolerate" the actions of the company.
Executive director Alan Boyter said he was "angry and frustrated" with Consort's repeated failures.
He 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.
“However patient safety is our number one priority and we have in place detailed contingency measures to ensure that patient safety is maintained at all times in every situation."
Earlier, the company said it had been "let down" by Consort and disciplinary proceedings had started.
Dr David Farquharson, medical director for NHS Lothian, said it had apologised to all the patients affected.
He said: "Planned maintenance was due to be carried out on the power supply after surgery was complete, but Private Finance Initiative (PFI) provider Consort failed to follow our critical procedures. Disciplinary proceedings are now under way.
"One patient was preparing to undergo a procedure but, because it had not begun, the operation was delayed until full power was available.
"Another procedure was being completed and our team of expert staff worked admirably using well-rehearsed business continuity plans to ensure that the patient did not come to any harm. Subsequent patients were not called until the situation could be fully investigated and we have apologised to those involved."
Before the threat of legal action by NHS Lothian, Consort said it was investigating the incident. After the threat of legal action, the company would not comment any further.
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."
The power cut is the latest in a line of problems caused by the private company.
For a week at the start of February, they did not tell the health board that smoke alarms in ten operating theatres were broken. In January, it came out the company had cleared 580 staff to work without carrying out background checks.
A report from the Healthcare Environment Inspectorate in September last year criticised the way the company and NHS Lothian worked together. The comments came after an inspectors found dirty wards and toilets.
Infrastructure and construction giant Balfour Beatty is one of the main investors in Consort Healthcare, which posted a turnover of £26.7m on its Edinburgh Royal Infirmary contract in 2010, with it making a profit of £8.9m - up on £6.1m in 2009.