A hospital in Oxford has suspended children's heart surgery after four recent deaths during operations.
The John Radcliffe Hospital said the fatalities involved some "very sick children" and happened in procedures during the past three to four months.
A spokesman said patients' families were being told and urgent cases were being transferred to other hospitals.
He said a review of each of the deaths would be carried out but other patients would not be put at risk by any delay.
Independent health regulator The Care Quality Commission (CQC) said it was not investigating the trust, but would monitor the trust's own review.
The hospital said the patients involved all had congenital heart problems, a general term for birth defects that affect the heart.
Other cardiology care of patients at the hospital will continue as usual.
The hospital is now in the process of contacting the families of 26 patients who were expecting to have surgery at the hospital in the near future.
In a statement, 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.
"This is high-risk surgery and the trust is taking the right steps in conducting this review."
"We are reviewing each of these patients to ensure that they are not put at risk by any delay in their treatment."
Roger Davidson, of the CQC, said: "We will monitor the situation extremely closely.
"It is reassuring that the trust has taken action to ensure the immediate safety of patients."
The hospital said surgery outcomes are "nationally validated" and Oxford had been "within normal outcome ranges".
The statement added: "This is one of the reasons why we felt it was necessary to take a close look at our service to ensure we continue to provide a high quality safe service for our children.
"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 in to the individual cases, to see if there is anything to learn from them."
'Urgently needed improvement'
It is not known how long the investigation will take but the entire treatment process for the patients will be examined, not just the surgery, the trust said.
In 2007 the hospital was criticised for its death rates among adult heart patients.
The Healthcare Commission began an investigation in November 2005 after it emerged that the number of trust patients who died between April 2002 and March 2005 after their first coronary artery bypass graft was more than double the national average - 4.01% compared with 1.83%.
Its report said that while rates at the John Radcliffe's cardiac surgery unit were "acceptable" improvements were urgently needed.
The hospital, opened in the 1970s, is Oxford's main accident and emergency site and also provides acute medical and surgical services.
It is situated in Headington, about three miles east of Oxford city centre, and is the largest of the Oxford Radcliffe Hospitals NHS Trust's sites.