The NHS may lack enough key staff to make a seven-day service work properly, leaked Health Department documents reportedly reveal.
The assessments, obtained by The Guardian and Channel 4 News, are said to show senior officials have voiced concern over the lack of detailed costings, risk assessment, and limited data supporting the policy.
Channel 4 News reported that one document, a "risk register" for the seven-day services programme, dated July 25, refers to the possibility that there will not be enough resources to meet the deadline for the "complete roll-out" of the policy.
The documents are also reported to state that a "workforce overload" could mean it may not be possible to find enough skilled staff "meaning the full service cannot be delivered", according to the reports.
Another document, regarding a meeting with the 7 Day Services Governance Group, references under key risks and issues: "The detailed costs of delivering in hospitals, including accurate estimates of additional workforce requirements are not understood early enough," according to Channel 4 News.
No advance impact assessments have been done on how seven-day services will affect GPs, hospitals, and urgent and emergency care, according to a further document called Building The Evidence Base, Channel 4 News said.
The leaked documents are said to express fears the policy may not deliver on its key aim of improving services at weekends, with one stating: "It is possible that the programme delivers the planned outputs, but this does not result in the desired change (delivering against the plan but missing the point)."
Health Secretary Jeremy Hunt has promised to provide a "truly seven-day service" by 2020, and the policy was a major source of tension during the bitter junior doctors' dispute.
British Medical Association head Dr Mark Porter said the documents underlined its own concerns about the policy.
"To see in black and white that the Government has not only ignored these concerns - and those of other leading healthcare organisations - but has also disregarded its own risk assessment's warnings about the lack of staffing and funding needed to deliver further seven-day services, is both alarming and incredibly disappointing," he said.
"If the Government wants to make more services available across seven days, then it needs to urgently address how it will staff and fund them rather than continue to mislead the public and brand doctors - who already work round the clock, seven days a week - as a roadblock to their plans."
Professor Jane Dacre, president of the Royal College of Physicians, told Channel Four News: "We would be very keen to give the same quality of care for patients seven days a week as we are able to do in five, but at the moment we are struggling to deliver good quality care over five days.
"So, to just expand to seven days and expect the quality to be the same, it's not surprising there are problems."
The leaked documents also expressed concern about the impact of Brexit on the high number of NHS staff from the EU. Labour shadow health secretary Diane Abbott described the situation as a "scandal" and said she would be contacting Mr Hunt to see if he had misled Parliament.
The Health Department insisted that a risk assessment was by definition a look at the worse case scenarios.
"Over the past six years eight independent studies have set out the evidence for a "weekend effect" -- unacceptable variation in care across the week. This government is the first to tackle this, with a commitment to a safer, seven day NHS for patients and £10 billion to fund the NHS's own plan for the future, alongside thousands of extra doctors and nurses on our wards," a Health Department spokesman said.