A "toxic" row between surgeons at a cardiac unit contributed to a higher mortality rate, according to a leaked document.
The report into the unit at St George's Hospital in London – seen by the Times – says the team was consumed by a "dark force" and patients were put at risk.
The average mortality rate nationally is 2%, but the unit recorded 3.7%.
The hospital said it was "taking action" and cardiac surgery at the unit was safe for patients.
St George's commissioned an external review of the department after an alert in April about "lower than expected survival rates".
Former NHS England deputy medical director Mike Bewick took on the task and wrote the report.
In it, he claimed internal scrutiny of the department was "inadequate" and the surgeons were split into "two camps" exhibiting "tribal-like activity".
Professor Bewick said: "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."
He added: "The surgical team is viewed as dysfunctional both internally and externally."
The review saw 39 staff interviewed and while they were shocked at the mortality rate, "most felt that poor performance was inevitable due to the pervading atmosphere".
It recommended stronger leadership and "new blood", and criticised the department for the "defensive approach" it took to the death rate.
The report also called for "radical solutions to breaking up the current surgical team".
A spokesman for St George's Hospital, in Tooting, south London, said the recommendations were being implemented "at pace", including the immediate relocation of all cardiac surgeons to a single-speciality practice.
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."
St George's has advertised for an additional consultant cardiologist to join the team.
London's Guy's and St Thomas's Foundation Trust will also send "on the ground leadership support" in the form of cardiac surgeons from 6 August.