Plans to improve food for government ministers and in prisons would mean hospitals get away with serving poor meals to patients, campaigners have said.
The Campaign for Better Hospital Food said Government proposals to introduce legally-binding standards in some areas will not go far enough to protect NHS patients.
It argues a situation is likely to arise where prisoners, ministers and civil servants are served better food than people in hospital.
Alex Jackson, co-ordinator of the Campaign for Better Hospital Food, has resigned from the Hospital Food Standards Panel set up by the Department of Health in protest at the plans.
The department does not back measures to bring in legally-binding standards for hospital food, with h ealth minister, Dr Dan Poulter, saying he believes the "best decisions" on hospital food are those taken locally by chefs and catering managers "rather than having centrally-imposed standards".
But such legal standards could be brought in for prisons and other areas under plans being considered by the Department for Environment, Food and Rural Affairs (Defra).
Mr Jackson said: "David Cameron's plans to change food served in prisons, schools and hospitals are shambolic and unfair.
"It is wrong for the Government to be proposing improvements to mandatory quality standards for prison food and to food served to ministers in government departments, yet refuses to extend the same quality standards to patient meals, even on a voluntary basis.
"The public, and especially patients, will be flabbergasted that - after 20 years of hospital food gimmicks - the Government still refuses to introduce legally-binding standards for hospital food served to patients.
"The Government must sort out this mess by making it compulsory for all food bought with taxpayers' money and served in our most important public institutions to meet the same higher quality standards, not just some of them."
Mr Jackson said the Government is aiming to improve its buying standards, which are mandatory for food served to civil servants, politicians, prison inmates and members of the armed forces.
They would require that more food is produced by British farmers, grown in season in the UK and cooked in a way that promotes good health.
At the same time, Mr Jackson said the Department of Health is planning to recommend food standards for hospitals to apply to patient meals on a voluntary basis.
He said some of the plans for hospitals look set to be "even weaker" than the mandatory standards that prison food must meet.
He expressed anger that the Hospital Food Standards Panel had been told to rule out legally-binding standards for hospital food from the start.
He said: "From the outset, I was dismayed that health minister, Dan Poulter, prohibited the Hospital Food Standards Panel from even considering the introduction of legally-binding standards for patient meals, if it saw fit to do so.
"Making this recommendation would have been the real breakthrough after 20 years of failed government voluntary initiatives to improve food for some of the most vulnerable people that the Government provides meals for.
"Having seen over 20 voluntary government initiatives fail to improve hospital over the past two decades at a cost of over £50 million to the taxpayer, I cannot in all good conscience continue to support yet another voluntary initiative, which I have every reason to believe will be ineffective."
A Department of Health spokesman said: "It is disappointing that the Campaign for Better Hospital Food has decided to completely misrepresent the facts and walk away from a project which will improve hospital food, set clear legally-binding standards, and has real power to change things for patients.
"The Hospital Food Standards Panel brings together experts who are passionate about improving the quality of hospital food throughout the NHS.
"Ministers have made it clear that the independent recommendations from the Hospital Food Standards Panel will help to improve legal requirements on hospitals to serve high quality food to patients."