Concerns that two "junior" surgeons were in charge of children's congenital heart surgery at Leeds General Infirmary (LGI) prompted the decision to suspend operations there, a key NHS adviser said on Saturday.
The hospital, which is at the centre of a long-running row over the future of its children's heart services, is carrying out an internal review after data suggested a death rate twice the national average.
But the accuracy of the mortality figures has been questioned after it emerged that the results were preliminary and scores of patients from Leeds had been omitted.
Campaigners have criticised the suspension of surgery and its timing, 24 hours after a High Court judge ruled that the decision-making process to close the children's unit was "legally flawed".
Professor Sir Roger Boyle, director of the National Institute of Clinical Outcomes Research (Nicor), which oversees mortality data across the NHS, said the figures were among a number of reasons he advised that the unit should be temporarily suspended.
He told BBC Breakfast that concerns were raised that two "relatively junior surgeons" had been left in charge of the unit, as well as from families of patients who claimed their requests to be transferred to other units were ignored.
"I was aware last weekend of other concerns being raised about Leeds, concerns raised by distinguished surgeons who don't work in the area," Sir Roger said.
"Concerns raised by families through the Children's Heart Foundation that they weren't being given the opportunity to be transferred to other units when they'd requested that.
"And I was also aware a senior surgeon was away on holiday, another surgeon was suspended and that left the service being offered to the public by two relatively junior, local surgeons.
"To have two relatively inexperienced people holding fort, without the ability for any senior advice, is a precarious situation in my view.
"It's a question of experience and fine balance between being able to offer a safe service and one that is precarious."
Doubts were cast over the early draft of mortality figures as it emerged that information from up to 150 cases from Leeds had been omitted.
But Sir Roger, the Government's former national director of heart disease, said further analysis which included those cases still showed a "gap" in mortality rates at the hospital.
"In fact, the data has been re-analysed with these extra cases and we still see this gap in mortality rate," he said.
"I would not have been able to sleep this weekend knowing that there were people who might have been operated on in a potentially unsafe environment."
Sir Bruce Keogh, the medical director of NHS England, said yesterday that the mortality figures were among a "constellation of reasons" to suspend operations, as well "disturbing" calls he received from two whistleblowers.
Sir Bruce said there had been "rumblings" among the cardiac surgical community for some time that "all was not well" in Leeds.
Dr John Gibbs, chairman of the steering committee for the Central Cardiology Audit Database (CCAD), which supplied the data, said he was "furious" that the figures had been leaked as they had not undergone the "usual rigorous checking process".
Greg Mulholland, Lib Dem MP for Leeds North West, said he was "stunned and appalled" by the suspension and called for Sir Bruce to resign.
In a statement on his website, Mr Mulholland said: "To have arrived in Leeds and done this, without warning, just one day after the decision to close the Leeds unit was proved in a court of law to have been unlawful, beggars belief.
"I believe that Sir Bruce Keogh should resign as he has both authorised this wholly unreasonable and deeply questionable action and also presided over the fundamentally flawed Safe and Sustainable review, which has proved an exercise in how not to effect major change to the NHS."
A campaign has been waged to save children's heart surgery at the LGI after the unit was earmarked for closure as part of an NHS plan to reorganise services across England into fewer, more specialised centres.
On Wednesday, Mrs Justice Nicola Davies allowed a challenge against the Leeds unit closure and declared the decision-making process "legally flawed".
She said there had been "a fundamental unfairness" in the consultation process.
The judge emphasised that she was not ordering that the whole consultation process had to be re-run.
Sir Roger told the BBC Radio 4 Today programme it was "pure coincidence" that the timing of the suspension came just a day after the High Court ruling quashed plans to close the unit.
He said: "The email I had from my analysts appeared as the judge was giving her views in the Royal Courts of Justice. It was pure coincidence.
"It is remarkable, but we had been working on this for over a year... trying to respond to the challenge of trying to separate out the hospitals on the basis of the mortality figures.
"The safe and sustainable review had to rely on a quality assessment of each centre and that was the point the judge found fault with."
Dr Elspeth Brown, consultant paediatric cardiologist at LGI, said the death rate figures were "simply wrong" and included just 180 operations out of more than 300 carried out at the hospital.
She told BBC News that the hospital was told on Thursday the decision to suspend surgery was based solely on the mortality data.
"We're very confident that our mortality figures are well within what would be expected," she said.
Dr Brown said the locum consultant surgeons in charge of the unit were "highly experienced and excellent".
"We need to put this bed, we need people to stop attacking us, and we need to resume surgery as soon as possible," she said.
The Archbishop of York visited the Leeds children's unit today to lead prayers with patients and staff.
Dr John Sentamu called for support for "all involved" at the unit as he arrived for a "pilgrimage of prayer and trust".
The archbishop has previously given his backing to campaigners battling to keep children's heart surgery in Leeds.
Visiting the unit in January, Dr Sentamu said the proposal to stop operations "does not make sense" when all children's services are on one site.
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