Senior civil servants in the Department of Health have warned that plans for a seven-day NHS could be undermined by a shortage of staff.
A risk assessment drawn up for health secretary Jeremy Hunt – and leaked to The Guardian and Channel 4 news – identifies 13 key risks to the plans, including "workforce overload".
The document also voices concerns that patients may not even notice any difference in care once the proposals have been implemented.
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It also points to the dangers Brexit might present to the plan, given that 55,000 NHS staff come from EU member states.
On wider workforce issues the register says there is a concern that "due to the scale of the change being delivered...it is not possible to fill all roles (consultants, doctors in General Practice and AHPs) with sufficiently skilled/trained staff to agreed timescales, meaning the full service cannot be delivered".
In a section called "scope creep" officials also warn that the aims of the proposals could change again.
The document says there is a risk that "the planned objectives and scope of the programme do not meet the expecations of No10/Cabinet Office, meaning that they may continue to change".
Shadow Health Secretary Diane Abbott called the revelations a "shocking indictment of the Tory government's plans", while Conservative former health minister Dan Poulter said the government could not "keep piling ever greater responsibilities on to an overstretched service without realistic resource and workforce to cope".
But the Department of Health defended the spirit of the proposals, while declining to comment on the specific contents of the leaked document.
"Over the past six years eight independent studies have set out the evidence for a 'weekend effect' – unacceptable variation in care across the week," a DH spokesperson said.
They added: "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 Whitehall source meanwhile pointed out that the aim of a risk register drawn up by civil servants is to highlight "all potential issues under a worst-case scenario to help the government develop robust plans".
The source said it was "entirely routine" for departmental governance groups to discuss "possible risks" to those plans.
A spokesperson for the Department of Health would not be drawn on whether a civil service leak inquiry would be launched into how the confidential advice entered the public domain.