The Government's spending review may not deliver a "workable" funding solution for the NHS, the head of the health service has warned.
Simon Stevens, the chief executive of NHS England, said spending reviews "usually come down to the wire".
Mr Stevens told the Health Service Journal (HSJ) that negotiations were continuing ahead of Chancellor George Osborne's announcement on November 25.
He told HSJ: "As of today, considerably more progress is going to be needed before we can say we have a genuinely workable NHS funding solution for 2016/17 and 2017/18, but spending reviews usually come down to the wire, so hopefully we'll get there by November 25."
The Government has committed to increasing NHS spending by £8 billion in real terms by 2020, but Mr Stevens wants this to be frontloaded.
He has other "tests" to meet his five-year forward view plan for the NHS, including investment and funding for social care services.
After taking part in the 2015 HSJ Summit event on Friday, Mr Stevens said: "I cannot confidently tell you today that we yet have a clear line of sight to those five tests being met [by the spending review]."
He added: "The Organisation for Economic Co-operation and Development has just reminded everyone that we're already incredibly lean in our country's health funding, overall, and on a per person basis.
"So the Government's funding commitment to 2020 was both highly welcome and absolutely necessary.
"But for the NHS, next year and the year after are where the rubber will really hit the road.
"The forward view maths was explicit that the health service needed frontloaded investment in 2016/17 and 2017/18 - to manage current pressures, kick-start service redesign, and unleash major savings later in the Parliament.
"We were also crystal clear that any further cuts in public health and social care would impose extra costs on the NHS over and above the minimum funding requirement."
Mr Stevens said NHS trust deficits, which are expected to hit £2 billion over the course of a year, should "fall substantially" next year, as long as there is a "realistic funding settlement".
He said there also needed to be progress on cutting the use of expensive agency staff and savings made through improved productivity.
However, he said NHS England and NHS Improvement would "not necessarily" expect deficits to be "eliminated altogether".
Shadow Health Secretary, Heidi Alexander, said: "Simon Stevens is absolutely right to issue this warning about the pressing need for the Government to address the deepening funding crisis in the NHS.
"It is now clear that unless there is an urgent and significant injection of money into the NHS, then standards of care will go backwards and the promise of a seven-day NHS will never be realised.
"The uncomfortable truth for this Tory Government is that the pressures facing the NHS are of their own making.
"Cuts to older people's care have piled the pressure on hospitals, and cuts to nurse training places have forced hospitals to drain resources hiring expensive agency staff."