Senior doctors have written to Prime Minister David Cameron calling for action over a legal loophole that enables drug companies to set "exorbitant prices" for drugs to treat rare diseases.
Pharmaceutical firms are obtaining licences for existing drugs under EU legislation and then hugely increasing the cost of them, they said.
Some NHS hospitals are now refusing to fund drugs that were previously available cheaply while the NHS faces paying millions of pounds extra a year for the same, or very similar, medicines.
The open letter, which is also addressed to Health Secretary Andrew Lansley, is signed by 20 consultants and is published online in the British Medical Journal (BMJ).
The doctors, who treat people with rare "orphan" diseases, said they were writing "to express our concern at an unintended effect of the European Union's regulations on orphan drugs.
The doctors said they had already written to the Department of Health for England and the Medicines and Healthcare Products Regulatory Agency about the issue.
"Legislation on orphan drugs, far from encouraging the development of new treatments for orphan diseases, is severely limiting the availability of existing treatments."
The doctors provide several examples of price hikes, including a drug to treat rare muscle diseases which used to cost £800 to £1,000 per patient per year.
In a BMJ investigation, published on Tuesday, Dr Sam Richmond, a consultant neonatologist at Sunderland Royal Infirmary, and a signatory of the open letter, said: "If drug companies are undertaking research where nobody else was interested - and some are - then a monopoly may be justified.
"But if it's a product already in use, they should clear off, or sell at a price comparable with the existing price."