The trust says it's grateful to cardiac staff for their hard work

But Morriston Hospital in Swansea says the inquiry is not related to the time off.

The trust said the investigation was related to concerns about "working relationships".

The rest of the cardiac unit has been operating as normal.

Routine heart operations have been cancelled, although emergency surgery will continue while staff are away from last Saturday to this Saturday.

We have a policy about dealing with bullying and harassment issues and that's not usually in the public domain Andrew Bellamy, Swansea NHS Trust

Patients who are affected are being contacted directly, but local GPs have raised concerns about the knock-on impact.

The trust said the investigation had been going on for about a month, and was continuing.

Trust director Andrew Bellamy said: "We can confirm there have been concerns raised by staff within that department.

"There is an investigation which has not yet concluded and it is possible that this could have contributed to the stress staff have been working under."

Mr Bellamy added: "We have a policy about dealing with bullying and harassment issues and that's not usually in the public domain."

In its earlier statement, the trust said the cardiac team had been under "considerable pressure" with overtime.

The statement said: "In order to allow staff to recover from these pressures it has been decided to limit activity to emergencies only for the next week.

"We are grateful to staff for all the hard work they have undertaken and hope that this will create some breathing space to allow them to recharge their batteries."

The unit includes a surgical ward and two operating theatres.

Plaid Cymru has said hospital staff have been working hard to cut heart surgery waiting lists, and the result was that staff were suffering some exhaustion.

Welsh Health Minister Jane Hutt said waiting times had come down as a result of assembly government investment.