Financial problems in the NHS are likely to cause a rise in waiting times in England, health chiefs believe.
Mike Farrar, the head of the NHS Confederation, said the difficulties could even lead to the 18-week limit for elective operations being broken.
He made the warning after feedback from senior NHS managers showed many feared it would get harder for patients to access care in the next 12 months.
But ministers insisted waiting times would remain low.
The NHS is currently meeting the 18-week standard overall.
However, the national figure masks the fact that a growing number of areas are experiencing difficulties.
The latest monthly statistics revealed a third of trusts were in breach of the limit for inpatients - double the number from a year ago.
This research by the NHS Confederation showed many health chiefs expected the situation to get worse.
The group, which represents managers, polled bosses working across the NHS in hospital, mental health and ambulance trusts. Private firms running dedicated NHS clinics were also quizzed.
They received 287 responses - only a third of the total they polled - but Mr Farrar said it still provided clear evidence of the pressures the NHS was under.
Nine in 10 described the financial situation as very serious with nearly half of them saying it was the worst they had ever seen.
While most felt this would not harm patient safety or the quality of care over the next year, 53% said patient access could get worse.
The NHS Confederation said this was because of increased rationing of services - there have been reports of restrictions being placed on everything from fertility treatment to hip and knee operations - and longer waiting times for treatment.
Speaking ahead of the NHS Confederation's annual conference which is starting in Manchester on Wednesday, Mr Farrar admitted there was a danger this could compromise the 18-week hospital waiting time limit.
"I think there is a risk. That is clearly what people are saying."
His warning comes just a month after David Cameron made a personal pledge to keep waiting times below 18 weeks as part of his drive to allay fears over the NHS reform programme.
Mr Farrar also called on politicians to stop attacking managers and called for them to show more "political courage" by supporting proposals to close hospital units where it was shown to be clinically necessary.
He said he had had experience of cases where MPs said they agreed with plans in private, but then opposed them in public.
"Managers need more support if we are going to deal with the difficulties ahead," he added.
Shadow health secretary John Healey said: "This is further evidence of widespread anxiety in the health service, as financial pressures impact on patient care and we start to see the NHS going backwards again under the Tories."
But Health Secretary Andrew Lansley said waiting times would continue to remain low and stable as promised.
He added: "Ultimately, our modernisation plans will safeguard the future of the NHS, improve care for patients, drive up quality and support doctors and nurses in providing the best possible care for their patients."
Meanwhile, a senior government official has attacked the government's overhaul of the NHS - despite the concessions made last month to water-down some of the proposals.
Sir Roger Boyle, the Department of Health's heart tsar, said the "mega reorganisation" could lead to people in the NHS taking their "eye off the ball" and therefore risk destabilising the service during the tough financial times.
Sir Roger said he was retiring soon partly because of his objection to the changes.
"I feel in my bones that the current plans are not correct."