Unison has called for an immediate halt to the government's Health and Social Care Bill that will reform the NHS and highlighted fears that private patients may get priority.
The public sector union is also considering participation in an NHS wide strike over pension reform being imposed on health workers.
It called the economic uncertainty, spiraling waiting lists and budget deficits a "lethal cocktail" meaning now was "the worst possible time to bring in a major, untried, untested reorganisation".
Christina McAnea, head of health for Unison, said: "If the Health and Social Care Bill goes ahead, the outlook for the NHS and patients looks bleak.
"The government's polices have already led to NHS patients waiting longer, often in great pain, for their operations.
"The Bill will make matters worse by taking the cap off the number of private patients that hospitals are allowed to treat.
"It will be an enormous temptation for cash strapped hospitals to boost their income by prioritising paying patients, pushing NHS patients even further down the ever-spiraling waiting lists," she added.
Unison noted that waiting six months or more for NHS treatment have increased by 61% in the last year and that the government's demand for £20bn in "efficiency savings" is leading to ward closures, staff cuts and rationing across the country.
"Even fourteen of the elite group of foundation trusts ended the last financial year in deficit, a grim warning for the future of NHS finances," McAnea continued.
"The economic uncertainty and budget deficits add to this lethal cocktail and should be obvious to the Government that now is not the time to bring in this massive, damaging NHS reorganisation."
Unison is one of several representative bodies, including the British Medical Association, Royal College of Nursing and British Dental Association, meeting to consider strike action over pension reforms for NHS staff.
