Children's heart surgery has been temporarily suspended at a hospital after the deaths of four young patients in recent months.
The John Radcliffe Hospital in Oxford has launched an investigation into the fatalities "to see if there is anything to learn from them", a spokesman said.
Parents of a total of 26 sick children currently waiting to have heart surgery have been contacted by the hospital trust. Those who need urgent treatment are to be placed with other hospitals.
The Oxford Radcliffe Hospitals NHS Trust spokesman 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."
The outcome of children's heart surgery is audited nationally and there has been no previous cause for concern, with the Oxford hospital falling "within normal outcome ranges", he said.
The spokesman added: "Any hospital might, at some time, have a run of deaths in a service that provides treatment for patients who are very unwell.
"However, it is right that we take a pause while we look into the individual cases, to see if there is anything to learn from them."
