Fewer than one in 10 nurses believe there is adequate staffing on NHS wards to deliver good-quality care, according to a poll.
The survey, of more than 1,900 nurses for the Royal College of Nursing (RCN), found 83% believe poor staffing compromises patient safety.
Just 7% of nurses said there are enough staff to care for patients properly, 80% think there are not and 13% are unsure.
Nurses also told the union they were coming under mounting pressure in the face of cuts, with posts left unfilled and recruitment freezes. There are fears that care may be "dumbed down" as a result of not having the right number and balance of staff.
The RCN is collating figures on the number of job losses at NHS trusts across England.
At the last count, 27,000 jobs had been earmarked for cuts. This includes redundancies, posts not being filled when staff retire or leave, and a diluting of the skill mix (balance of fully qualified staff with less-qualified assistants).
Of those questioned in the survey, 46% said that there had been unfilled vacancies in their workplace for more than six months, while 40% said there was currently a recruitment freeze.
Dr Peter Carter, chief executive of the RCN, said the Government's reforms of the NHS and the drive to find £15 to £20 billion annually in "efficiency savings" added to the pressure.
"The results of our survey act as a reality check for those saying that cuts aren't biting in the NHS," he said. "It is deeply worrying that some nurses are telling us that they do not have enough staff to deliver quality care and that safety could be compromised."
A Department of Health spokesman said: "The NHS must still find efficiency savings but we have been clear that every penny saved - including a 45% reduction in management costs - will be reinvested to support front line services and improve quality."