A hospital at the centre of a row about the streamlining of children's heart surgery units has been forced to restrict the number of children's cardiac operations.
The move comes after the health watchdog found that Bristol Royal Hospital for Children has inadequate staff levels to care for patients on a cardiac ward.
The Care Quality Commission (CQC) said said it had given the hospital a formal warning for not having appropriate staff numbers on a ward caring for babies and children with heart problems.
Earlier this year, the hospital was selected to be one of seven specialist institutions to provide paediatric heart services for patients across England and Wales.
NHS officials opted to keep seven units running and decided to close the surgical units at Leeds General Infirmary, Glenfield Hospital in Leicester and the Royal Brompton in Chelsea in west London despite numerous campaigns to keep the units at the hospitals.
Officials made the decision after an NHS review concluded that expertise was spread too thinly and should be concentrated in fewer hospitals.
The Safe and Sustainable review followed the landmark inquiry into children's heart surgery at Bristol Royal Infirmary between 1990 and 1995, where up to 35 children and babies died as a result of poor care.
After the inquiry, it was recommended that paediatric cardiac units should be set a target for the number of operations per year, and surgery be concentrated in a few specialist centres to ensure quality of care.
In July, the Joint Committee of Primary Care Trusts (JCPCT) of England decided that Bristol would be one of the specialist centres.
But today the CQC said that, during an unannounced inspection, it found there were not enough qualified, skilled and experienced staff to meet patients' needs on one of the wards at the hospital.
The CQC said it issued a formal warning to University Hospitals Bristol NHS Foundation Trust, which runs the hospital, following the inspection on September 5.
The watchdog said it visited the hospital after the families of two patients raised concerns about care on Ward 32 - a 16-bed unit which cares for children up to the age of 18.
The trust said that, since the warning, the hospital has reduced the number of beds on the ward to 12.
It has also decided to reduce its programme of cardiac surgery in line with the new bed capacity.
Inspectors found that the hospital had been failing to meet three essential standards of quality and safety covering staffing levels, staff training and support, and the overall care and welfare of patients.
They found that the ward was caring for children in need of "high dependency care" but staffing levels were not appropriate to look after such patients, who require closer observation and monitoring than on an ordinary children's ward.
The CQC said staffing levels were in line with those found on a paediatric general ward - one nurse for every four children over the age of two during the day - but they should have been one nurse for every two children for high-dependency care.
Some of the staff did not have the appropriate experience to care for children in need of high-dependency care, the CQC added.
Several members of staff told inspectors they were worried about the lack of specialist training for doctors, registered nurses and healthcare assistants in children's cardiac care or high dependency care.
Ian Biggs, deputy director of CQC in the South, said: "Everyone seems to agree that there should have been more staff on duty looking after some very frail children.
"Doctors and nurses told us that the current staffing levels were having an impact on the care and service being provided, with checks missed or reduced at times, and not enough time to communicate with families.
"We found that, on a number of shifts, staffing levels on Ward 32 fell below these inadequate levels and, to make matters worse, some staff did not have the appropriate experience to care for patients with high-dependency needs.
"Since our inspection, University Hospitals Bristol NHS Foundation Trust has taken steps to reduce the number of cardiac beds and ensure a higher ratio of nurses to children.
"While the immediate effect may be fewer heart operations in Bristol, I am sure that the trust will work with commissioners to plan for the long term how it provides a safe service which fully meets its role as the regional centre for children's heart surgery."
Robert Woolley, chief executive of the trust, said: "No family should leave the Bristol Royal Hospital for Children believing that we did not provide the best care possible for their child.
"I am deeply concerned that some families believe that we have let them down and will continue to ensure that we address their concerns and bring forward our plans to develop a high-dependency unit."
Mr Woolley said that, as a regional centre, the hospital sees some of the sickest children with the most complex needs from across the South West of England and South Wales.
He said that, following the CQC inspection, a warning notice was issued to the trust in relation to staffing levels and as a consequence the plans to establish a cardiac high-dependency unit have been brought forward.
Mr Woolley added: "In the meantime, we've also created two dedicated high-dependency beds on the paediatric intensive care unit where cardiac children who require high-dependency care will be looked after.
"We have also improved the nurse-to-patient numbers on the cardiac ward.
"As a result of these actions, I believe we have addressed the issues leading to the warning notice and I would welcome a return visit from the CQC to confirm this."
The Health Secretary has since ordered a review into the JCPCT's decision.
Jeremy Hunt has asked the Independent Reconfiguration Panel to carry out a review into the decision to close the units in Leeds, Leicester and at the Royal Brompton.