Some health trusts are banning medicines approved for use on the NHS because they are too expensive, research suggests.
Drugs to treat conditions including cancer, heart disease, asthma, diabetes and epilepsy have been blacklisted, with GPs told not to prescribe them.
This is despite the fact the medicines have been approved by the National Institute for Health and Clinical Excellence (Nice) for use on the NHS.
A freedom of information investigation by GP newspaper found one in four primary care trusts (PCTs) surveyed had blacklisted Nice drugs.
A total of 71 PCTs in England responded to the request for information. Some 33 PCTs had a blacklist of drugs, of which 18 had Nice-approved drugs on the list.
Drugs on the list are "not recommended for use" or "not prescribable" because they are deemed too expensive or not effective enough.
The NHS Constitution gives patients the right to receive any drug recommended by Nice if their doctor believes it is clinically appropriate.
But 14 drugs recommended by Nice and a further five drugs backed in Nice clinical guidance have been blacklisted by PCTs, according to the research.
Dr Chaand Nagpaul, from the British Medical Association's GPs committee, said: "If an area is blacklisting a Nice-approved drug, it is breaking its duty of care. It is falling foul of the whole purpose of Nice recommendations."
A spokesman for the Department of Health said: "There's no excuse for PCTs to be denying patients drugs that are recommended by Nice and that their doctors believe they need. PCTs have no legal ability to prevent a GP prescribing a particular drug."