A hospital at centre of a long-running row over the future of its children's heart services has unexpectedly suspended all congenital cardiac surgery on youngsters.
Leeds General Infirmary said the temporary measures was being taken to allow a internal review to take place.
NHS England said checks needed to be made to ensure the unit is operating safely.
Sharon Cheng, from Save Our Surgery - the group which is co-ordinating the fight to keep children's heart surgery in Leeds - said: "We're mystified. We don't know of anything that could justify this step."
Maggie Boyle, the chief executive of Leeds Teaching Hospitals NHS Trust, said: "Following discussions earlier with senior representatives from NHS England and the Care Quality Commission the trust has agreed to carry out an internal review, independently validated and supported by external experts.
"This will look at all aspects of congenital cardiac surgery for children undertaken at the unit in Leeds.
"We have taken the decision to temporarily pause children's congenital cardiac surgery and associated interventions while this review is conducted, a process we would aim to complete in around three weeks.
"We apologise to parents and families who will be affected during this time, and can assure them we always put the safety of our patients first.
"It is really important to us that the review is done as speedily and comprehensively as possible which, of course, we hope will show the services in Leeds to be safe.
"We are confident in the quality of the care provided by our staff and hope they will bear with us during this difficult time."
Sir Bruce Keogh, the medical director of NHS England, said: "The trust has taken a highly responsible precautionary step.
"Some questions have been raised by the trust's own mortality data and by other information.
"It is important to understand that while this information raises questions, it does not give us answers.
"But it is absolutely right not to take any risks while these matters are being looked into.
"The priority must be the safety of children. I hope that Leeds will shortly be in a position to restart children's heart surgery secure in the knowledge that everything is okay."
A decision to stop children's heart surgery at the Leeds hospital was quashed by a High Court judge on Wednesday, the BBC reported.
An NHS review had previously recommended that the surgery should stop at hospitals in Leeds, Leicester and London to focus care at larger premises. But judge Mrs Justice Nicola Davies said there had been "a fundamental unfairness" in the consultation process.
Lawyers involved on either side of the challenge are now considering the full impact of her ruling - and there is disagreement over the extent to which reconsultation and reconsideration must take place.
Save Our Surgery said it believes the ruling means that all the hospitals that face losing their children's units will now be able to fight again retain them.
But health service legal experts say the new consultations will be more restricted and will focus more narrowly on the issues in the Leeds case.
A spokesman for the Care Quality Commission said: "We support the Trust's decision to carry out a review of their children's heart surgery unit and to put existing activity on hold pending the outcome of the review.
"We are in close contact with the trust to ensure effective arrangments are in place to protect the safety and welfare of patients during the period of of the review. We are monitoring the situation extremely closely and will not hesitate to take regulatory action if we believe this is required."
Earlier this month, the CQC confirmed it had received claims that the uncertainty over the future of children's heart surgery at the LGI was harming outcomes because of a reluctance to refer some patients to the Freeman Hospital, in Newcastle.
The Freeman Hospital's unit would be spared if the original NHS re-organisation plan continues.
The LGI strongly refuted "any suggestion that we would act improperly either by restricting referrals or by failing to carry out surgery where either of these actions was the right thing to do".