The health watchdog has been forced to take action against a hospital trust which performed a surgery on the wrong patient.
Within a month, Cambridge University Hospitals NHS Foundation Trust performed four botched surgeries, including leaving instruments inside two patients and performing surgery on the wrong person.
Surgeons also operated on the wrong part of the body of another patient, Monitor said. The regulator said that since September 2011, eight "never events" - instances which are never supposed to happen - have happened at the trust, including four between September and October last year.
Monitor said that it was stepping in because the trust has also failed to give cancer patients treatment in the recommended time. There has been "successive failure" to meet healthcare targets including waiting times for cancer treatment and accident and emergency performance, Monitor said.
A spokeswoman said the regulator has taken action to "improve the quality and organisation of healthcare" for patients.
Monitor said that the "wrong person surgery" related to a patient who had the wrong surgical lens fitted. Several patients were attending the hospital for similar operations at the same time, a spokeswoman said. The wrong lens was fitted to the wrong patient and then quickly removed - the patient did not suffer any ill effects, a spokeswoman said.
Monitor has also raised concerns about the trust's financial performance and the board's capacity.
Monitor's managing director of provider regulation Stephen Hay said: "This is not the first time we have called the Trust in to explain itself. We are disappointed that the Board has not resolved these issues. We note the trust has a new chair and will shortly appoint a new chief executive. We expect them to demonstrate they are getting the trust back on track as quickly as possible."
The trust, which runs Addenbrooke's and the Rosie hospital, has been ordered to commission a "governance and effectiveness review". A board level "experienced turnaround expert" should also be appointed, Monitor recommended.
Chairman Jane Ramsey stressed that the Trust was determined to reverse the situation and improve performance. She said: "We take Monitor's concerns very seriously and we are determined to reverse the situation as soon as possible."