One of the biggest NHS trusts in the country has been placed in special measures after inspectors deemed it "inadequate".
Cambridge University Hospitals NHS Foundation Trust, which runs Addenbrooke's Hospital, has been told it requires improvement to ensure services are safe and effective.
While staff were found to be caring, inspectors from the Care Quality Commission (CQC) found staff shortages and long-standing "serious" problems that had been ignored.
In maternity, inspectors raised "serious concerns", including a low midwife-to-birth ratio, and noted wards were closed regularly.
In one example, high levels of nitrous oxide - also known as laughing gas, used for pain relief in childbirth - were detected in the Rosie Birth Centre.
Experts have linked high levels to feelings of dizziness, while other risks include unconsciousness or death if there is a lack of oxygen.
The CQC inspectors said senior managers had been aware of the high levels of gas for more than two years. However, the only action taken was to advise staff to open windows where possible.
In their report, the inspectors also said equipment in the maternity unit was old.
Best practice guidelines were not always followed, including on continuous foetal heart rate monitoring during labour, and the risk of blood clots.
Across the entire trust, inspectors said staff shortages meant there was not enough cover on wards, including critical care, and employees were moved from ward to ward to make up shortfalls.
This meant staff sometimes worked in areas where they lacked training, while some rotas had empty spaces and others were filled with agency workers.
Inspectors also noted a "disconnect between what was happening on the front line and the senior management team".
Many patients were waiting for follow-ups, the CQC found. During the inspection, there was a backlog of 227 ophthalmology and 233 dermatology patients waiting for a call back and a total of 605 across all specialities.
The pressure on surgical services also meant routine operations were often cancelled and patients were waiting longer than the Government's 18-week target for treatment.
Cambridge University Hospitals NHS Foundation Trust, which has more than 1,000 beds, is already under scrutiny by Monitor over its financial dealings.
The trust is running a deficit of £1.2 million per week and is predicted to overrun by at least £64 million this year.
Former chief executive Keith McNeil, who resigned last week, has defended Addenbrooke's as "phenomenal".
"Everywhere across the country people would be very envious of the sort of results we get day in day out," he said.
"People's lives are saved every day by that hospital. I cannot see why anybody would want to describe it as inadequate."
Asked before the report was published whether the assessment by the CQC was wrong, he told BBC Radio Cambridgeshire: "I believe so."
He said he did not think there was "any sane or rational interpretation of the word 'inadequate'" that would describe any aspect of the operations at Addenbrooke's.
The hospital's chief finance officer, Paul James, also quit ahead of the CQC report.
The CQC's chief inspector of hospitals, Professor Sir Mike Richards, said patient safety and welfare was "placed at risk" at the trust because some patients were being looked after by staff who were not properly trained.
He added: "Staff were hard-working, passionate and caring throughout the trust, prepared to go the extra mile for patients, but having to swim upstream against the pressures they faced."
Trust chairwoman Jane Ramsey said: "I would like to say sorry to our patients for a lack of effective systems and processes across our trust, which has led to the Care Quality Commission today rating our hospitals as inadequate.
"We take this, and being placed in special measures by our regulator Monitor, very seriously.
"Part of Monitor's enforcement action means we have a number of clearly defined quality, financial and governance failings to rectify as soon as possible. We will take rapid action to address these concerns and maintain our record of safety and high-quality care.
"The care our staff provide has been rated as outstanding and we are very proud of them, and we continue to have a strong track record on safety, clinical excellence and introducing ground-breaking treatments."
Interim chief executive David Wherrett said: "Against a backdrop of increased demand and tighter resources, our services continue to be recognised nationally and internationally for their safety and patient outcomes.
"Patients are less likely to die here, be harmed, or catch a hospital-acquired infection than at almost any other trust in the country."
He said he wished to reassure patients that while work was ongoing to address the CQC's concerns, they will remain "in safe hands, receive outstanding care from compassionate staff and be treated by leading clinicians".
The CQC also put another trust, East Sussex Healthcare NHS Trust, in special measures.
Inspectors rated the trust as inadequate and pointed to a range of issues, including low staffing levels in surgery, maternity and pharmacy services.
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