The NHS faces "serious challenges in maintaining standards of care", with senior managers worried about having the right levels of staff, according to a new report.
The NHS Providers study of 158 chairmen and chief executives from 125 NHS trusts also found concerns about NHS finances and the current state of mental health.
More than half (53%) of those surveyed were confident they were currently able to provide high-quality care, down from 60% in November, while 8% said the care offered by their trust was very high quality.
But more than a third (37%) thought the care given to patients was average, while 2% said it was low quality.
The report said: "NHS trusts are experiencing record levels of demand for their services.
"At the same time, patients have higher and more complex needs.
"Many trusts are now running at capacity levels beyond the recommended norm and levels in other advanced western health systems."
The report said for the first time, the NHS is missing all four of its key targets: 75% ambulance response within eight minutes, 95% A&E four-hour waiting times, 92% 18-week elective surgery and 85% seen within 62 days of GP referral.
The study said the "NHS is in the middle of the longest and deepest funding squeeze in its history".
While the extra funding promised in the Conservative manifesto is welcome, spending on health will still significantly fall as a percentage of national wealth until 2022/23, it added.
The survey found that 57% of managers questioned were worried or very worried about their ability to maintain the right numbers of staff, both clinical and non-clinical, to deliver high-quality care.
Meanwhile, a quarter expected their finances to improve over the next six months. A third expected them to deteriorate.
The study also looked at mental health, with a stark warning that the Government's commitment to tackle injustices faced by people with mental health issues is being undermined.
It said the impact of increasing demand, too few staff and the failure of funding to get through to the frontline means core mental health services are being overwhelmed.
The poll included 43 chairmen and chief executives from 37 mental health trusts, of which more than 70% expected demand to increase this year.
Fewer than one in three felt confident they had enough staff to deliver existing services let alone extending or creating new services.
The report said that, in particular, trusts are struggling to recruit enough mental health nurses and psychiatrists.
And 80% said cash intended for mental health from a national level is still not getting though the frontline and is partly being swallowed up in Clinical Commissioning Group (CCG) budgets.
Saffron Cordery, director of policy and strategy at NHS Providers, said: "Having the right staff with the right skills in the right place is the only way to improve mental health services on the ground.
"But mental health trust leaders are struggling to find sufficient staff to deliver their current services, let alone find new staff to extend, transform or innovate services.
"And too often, trust leaders report that any extra funding is just used to fill existing gaps or to manage current demand, not improve service quality or access.
"Unless action is taken to address these areas of concern then the Government's ambitions for transforming mental health care will not be met."
Professor Wendy Burn, president of the Royal College of Psychiatrists, said: "When 80% of the people who run NHS mental health services are worried that they will not have enough money to deliver the Government's commitments to improve mental healthcare, we should all listen.
"It is deeply concerning that so many providers fear that they will not have resources both in terms of staff and money to deliver the promised improvements for mental health patients over the coming years."
The report also said that increasing awareness of mental health, including campaigns such as Heads Together, which is led by the Duke and Duchess of Cambridge and Prince Harry, is understandably leading to more demand for services.