Discrimination against the elderly in the NHS is so pervasive that half of GPs fear they will themselves be denied appropriate care as they grow old, a survey suggests.

The research was done last month by the NOP polling organisation for Age Concern. Two hundred GPs, representative of the 35,000 in Britain, were interviewed face-to-face.

Three-quarters of doctors said patients are denied treatment because of age, counter to reassurances that treatment is based on clinical need. A third said they knew of age limits for heart bypass surgery and kidney dialysis and one in seven said elderly were denied admission to coronary and intensive-care units. Forty per cent said they would be worried about a frail elderly relative going into an NHS hospital.

The survey adds to evidence of NHS "age rationing". In November Age Concern received 1,000 letters from people who said they had been fobbed off, under-valued or abused by the service. A month later it highlighted claims that patients were being starved in a form of involuntary euthanasia.

Last month Age Concern amassed a dossier of 100 cases in which people had had "do not resuscitate" orders secretly added to their notes in breach of guidelines about obtaining consent. Calls flooded in after a stomach-cancer patient, Jill Baker, 67, found the instruction had been included in her notes without her knowledge. She received an apology.

In another case, Renee Mucklejohn, 87, who was in intensive care because of complications after bowel surgery, was moved to the general ward of Royal Berkshire Hospital 13 hours after her son had been told to prepare for the worst.

Ian Mucklejohn said: "I had been there all day with my mother and managed to bring her round. I went home ... and at 12.30am the doctor phoned to say they needed to admit a 30-year-old patient and that meant they had to lose their fittest patient. That patient was my mother." He was offered the choice for his mother of a general ward bed or a move to an intensive care unit in Redhill or Milton Keynes. He left the decision to the doctor and his mother was moved to a general ward, where she died next day. Mr Mucklejohn said: "Some hours after having been pronounced the fittest patient in the intensive care unit she was dead. If the decision to move her was made on medical grounds, why was I given the age of the person coming in?"

He is pursuing a formal complaint against the Royal Berkshire, which said: "The decision to move her was based on medical grounds, not age."

A theme in the Age Concern survey was of older people "made to feel guilty about being a burden on the state" and of GPs "having to apologise" for requesting their admission to hospital. Sally Greengross, director general of Age Concern, said: "This survey confirms that age discrimination is the scourge of today's NHS."

In a separate study, Royal College of Physicians researchers found lung cancer patients over 75 were less than half as likely to receive potentially life-prolonging surgery as those under 65.