Unison said the Government's squeeze on public spending was "dragging" the health service back to the 1980s and 1990s when the NHS was "starved" of funds.
A survey of more than 8,000 staff showed that hospitals were freezing recruitment, cutting posts and outsourcing or cutting services.
Unison said its study revealed a front line under pressure, with more patients treated by fewer staff.
Unison's head of health Karen Jennings said: "What is truly distressing is that the survey clearly shows how spending cuts are already threatening to damage the quality of patient care.
"The Government is turning back the clock and dragging the country back to the dark days of the 80s and early 90s when the NHS was starved of money.
"Back then, patients who could afford it, paid up to avoid being at the end of a very long waiting list or being treated in poorly maintained hospitals.
"Government cuts threaten to undo and reverse the benefits of all the investment and hard work that has gone into turning the NHS around over the past 13 years.
"We have been able to train our own nurses instead of scouring third world countries to fill shortages."
A Department of Health spokesperson said: "The coalition Government has made an historic commitment to increase NHS spending in real terms in every year of this Parliament.
"But the demand on the NHS is so great that in order to sustain and improve services, we need to make every penny count.
"The efficiency drive is about cutting waste and bureaucracy."