A “toxic” feud between two rival camps at a troubled heart surgery unit left staff feeling that a high death rate was inevitable, according to a leaked report.

St George’s hospital heart unit was consumed by a “dark force” and patients were put at risk by a dysfunctional team of surgeons, an investigation concluded last month. The damning review was written by former NHS England deputy medical director Mike Bewick in response to high mortality rates at the hospital.

He found that the facility in Tooting, south London had a cardiac surgery death rate of 3.7% – above the national 2% average. Internal scrutiny was said to be “inadequate” and the department was riven between “two camps” exhibiting “tribal-like activity”.

Bewick’s review was quoted as saying: “Some felt that there was a persistent toxic atmosphere and stated that there was a ‘dark force’ in the unit. In our view, the whole team shares responsibility for the failure to significantly improve professional relationships and to a degree surgical mortality.”

Conversations with 39 staff revealed they were shocked by the death rate, but “most felt that poor performance was inevitable due to the pervading atmosphere”.

The independent reviewer examined “disturbing and often difficult information”, concluding that the unit faced an “existential threat” because staff and patients would go elsewhere if problems persisted.

“The surgical team is viewed as dysfunctional both internally and externally,” he said. Stronger leadership and new blood were called for, while the “defensive approach” the unit took to death rate data was criticised, according to reports. The review was said to have called for “radical solutions to breaking up the current surgical team”.

A spokesman for St George’s said Bewick’s recommendations were being implemented “at pace”, including the immediate relocation of all cardiac surgeons to a single-speciality practice.

The investigation was commissioned in June after the National Institute for Cardiovascular Outcomes Research issued an alert over the unit’s low surgery survival rates.

The hospital spokesman said: “We would like to stress that the cardiac surgery service we provide for patients is safe. However, it is very clear that major and urgent improvements are required, which we are already taking action to deliver.”

A spokesperson for the Royal College of Surgeons and the Society for Cardiothoracic Surgery, said: “Cardiac surgery within the UK is amongst the safest in the world and St George’s has always shown that early survival for their patients is within what is regarded as the UK standard.

“It has never been below the required standard for the UK. All the other hospitals in the UK have shown that early survival after cardiac surgery is at the required standard, with only St George’s triggering the early warning alert over the last four years of surgery.

“Nevertheless, the independent review has highlighted serious concerns so it is still crucial that St George’s address the findings of the report so that patients can be sure they are receiving the best possible care.”