"Hostility" between surgeons at a beleaguered hospital trust led to them needing to take part in mediation to try and resolve their issues, a new report states.
Health inspectors have ordered immediate action to improve services at the cardiac surgery unit at St George's University Hospitals NHS Foundation Trust in south-west London.
A new Care Quality Commission (CQC) report into the unit states there was an atmosphere of "hostility" between surgeons.
Top physicians were described as having "strong" personalities, and were unable to work together effectively.
Culture in the surgical unit was described as "tribalism" by one consultant.
Staff told CQC inspectors that the cardiac surgical team "had not worked effectively for several years".
Surgeons even had to take part in mediation to try and resolve their issues.
While this showed improvements initially, previous "poor behaviour" soon resumed, the CQC's report states.
Inspectors wrote: "Consultants did not work collaboratively, share responsibility or resolve conflict in a constructive and timely manner.
"There were high levels of mistrust amongst clinical colleagues which contributed to the poor culture within the service.
"The culture within the cardiac surgical service did not always encourage openness and honesty and we found that staff did not work together to ensure delivery of high-quality, safe and effective services that put patients at the centre."
Inspectors said they were "not assured there was credible and effective leadership or managerial oversight at service, divisional or trust level" to address the issues.
They conducted a focused inspection of the cardiac surgery unit over three days in August and September after concerns were raised about mortality rates.
But the latest data shows mortality rates have improved since the report was published.
The inspection was also launched following concerns over culture, governance and leadership.
Inspectors wrote: "We found the cardiac surgery unit was going through a significant transition. Local governance and leadership were weak and were being revised to help improve the service.
"The culture was poor. Consultant surgeons mistrusted each other, as well as cardiologists, anaesthetists and senior leaders."
In September it was announced that the most complex heart surgery cases at St George's were to be diverted to other hospitals to "enable improvements to be made".
And hospital bosses said they had asked external regulators to oversee work being done to deal with "long-standing issues" relating to the cardiac surgical service.
The trust has been ordered to make a series of improvements including a call to address cultural issues within the service and to resolve issues over leadership.
CQC's chief inspector of hospitals Professor Ted Baker said: "Issues such as weak leadership, internal unrest and multiple electronic patient record systems are just some of the problems affecting St George's Hospital cardiac surgery unit."
Jacqueline Totterdell, chief executive of St George's, said: "The service experienced significant challenges earlier this year, and it will be reassuring to the communities we serve that the CQC has confirmed it is safe.
"Of course, this does not change the fact that significant improvements are required - and significant progress has been made since the CQC's visit back in August.
"I am grateful to many of our cardiac surgery staff for the commitment and dedication they have shown - they deserve a huge amount of praise during what was a difficult period for the service."
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