Concerns about children's heart surgery units were raised prior to the decision to suspend such operations at a leading hospital, it was revealed today.

The John Radcliffe Hospital in Oxford has cancelled all paediatric heart operations following the deaths of four young patients in as many months.

The deaths of the "very sick children", who had congenital heart defects, happened either during or after surgery. Parents of 26 children due to undergo surgery there are now being told they will have to have treatment at one of 10 other units in the UK.

Leslie Hamilton, President of the Society of Cardiothoracic Surgery, said they had been "concerned for some time" about the number of specialist units, however.

He said: "There are 25 to 30 surgeons involved in the whole country. We feel they are spread too thinly across those units.

"Everyone is working hard to provide a good quality of care but we don't think it is sustainable."

A review is currently under way to determine whether or not to pool the resources of those units, Mr Hamilton said. The publication of a NHS National Commissioning Group report into the matter is due later this year.

Mr Hamilton added that lessons had already been learned from the 1998 inquiry into the high rate of child deaths after heart surgery at the Bristol Royal Infirmary.

He welcomed the Oxford investigation, saying: "Statistically you can get a run of deaths occurring together. I think what the Trust has done here is entirely appropriate.

"They need to determine if this is a statistical run or if this is a question of quality of care.

"We learned a lot of lessons from the Bristol inquiry. They did things to improve the problem, but the feeling was that it went on for too long.

"I don't think anyone would allow that to continue now. We have learned these lessons from the past, which is why this decision has been made."

The Care Quality Commission has said it is closely monitoring the situation in Oxford.

Deaths after children's heart surgery at the hospital fall within normal ranges and there has been no previous cause for concern. About 100 children undergo heart surgery at the hospital each year.

A spokesman for the Oxford Radcliffe Hospitals NHS Trust said: "We have temporarily suspended the paediatric cardiac surgery service while we investigate concerns that have been raised.

"Our investigations are likely to include an external review of the deaths of some very sick children (four children) who underwent surgery at the John Radcliffe Hospital in the last three to four months.

"The families of children awaiting cardiac surgery are being contacted and we will be placing those patients who need to be seen urgently with other hospitals.

"We are reviewing each of these patients to ensure that they are not put at risk by any delay in their treatment."

Health Secretary Andy Burnham told Sky News parents should be reassured by the speed at which an inquiry was launched.

He said: "Obviously, the hospital have done the right thing by acting quickly to suspend further surgery.

"I think the important thing to say is this is very high-risk surgery. It's often dealing with some very poorly children indeed.

"I think it's important to put that context out."

The hospital has previously been criticised for mortality rates following adult heart surgery.

An investigation by the Healthcare Commission into deaths from 2002 to 2005 found there was a higher than average death rate for patients undergoing a coronary artery bypass graft, the most common type of heart surgery.

The report published by the Healthcare Commission in 2007 concluded that death rates fell within "expected" levels but said improvements were desperately needed.

A 2000 report also criticised "many aspects of the service", the Healthcare Commission said at the time, adding: "For some years there have been concerns about the rates of mortality for patients undergoing cardiac surgery at the John Radcliffe Hospital."