The NHS may not have enough staff to make a seven-day service work, leaked Health Department documents have reportedly revealed.
The secret papers, obtained by The Guardian and Channel 4 News, are said to detail a string of dangers in implementing the plan.
As well as too few staff, there is also not enough money to deliver the government's promise on time.
And patients may not notice any difference even if it happens, according to the reports.
The confidential papers, drawn up by Health Secretary Jeremy Hunt and his team, are said to have uncovered 13 major risks to the plan.
The biggest danger is reportedly "workforce overload" - which could mean there are not enough skilled staff "meaning the full service cannot be delivered".
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.
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."
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.