A drug company whose treatment for a rare enzyme disease costs more than £60,000 a year for the average adult patient faces big fines for allegedly abusing its dominant position in the market.

The office of fair trading believes Genzyme, makers of the only effective treatment for Gaucher disease, is breaking competition law and is threatening the US-based company with penalties that could reach 10% of its £54m a year British turnover.

The drug, called Cerezyme, uses ovary cells from Chinese hamsters to make an artificial enzyme for patients whose bodies cannot process products from their cells and who therefore develop deposits of the material around their livers, spleens, bones and joints. It is not suitable for all types of the disease, which affects up to 40,000 people worldwide, and only about 3,000 in all may be on the Genzyme drug.

Some patients with the genetic disorder show few signs of the disease but it can lead to severe disability and death. Genzyme charges the NHS a blanket price for the drug, which not only covers the treatment, injected intravenously, but also delivery to patients' homes and nursing support. The OFT believes this effectively ensures that only Genzyme, or a company under contract to it, can provide such services since any independent provider would have no margin for profit. The proposed finding has still to be confirmed.

The company, which says its drug is used on about 185 of the 250 to 300 people with Gaucher disease in Britain, denies any infringement of the law. It used to employ a company, Healthcare at Home, to supervise delivery and administration of the drug but now has its own in-house service.

Dominic Moreland, director of Genzyme Homecare, in Oxford, said the company intended to take a "robust position against the proposed decision" of the OFT and questioned whether the penalties, if imposed, would be near 10% of Genzyme's British turnover. They were more likely to amount to 10% of turnover for Cerezyme, a figure he declined to divulge.

The service to patients at home varied from simple delivery of the drug to setting up the infusion, administering it, and recovering needles and equipment for safe disposal.

"We feel we can best provide patients with the highest quality of care if we are also able to offer homecare services ourselves. Genzyme has made a very substantial investment in order to achieve this," he said.

The company planned to introduce further therapies for similar diseases but its home services would be specialised and would not threaten companies providing more general nursing services.

Only four centres, two for children, specialise in the disease in Britain. Atul Mehta, who runs the Gaucher centre at the Royal Free hospital in London, said the average adult patient received about 800 units of Cerezyme in each fortnightly infusion. That would cost the NHS £2,380.

"This therapy works, patients improve and it is cheap at the price. But many of us are uncomfortable one pharmaceutical company should be able to deal directly with patients."

He said this might be bad for competition, which should stimulate new therapies using different technologies.

Another company, Oxford Glycosciences, has just won European approval for a tablet to treat Gaucher disease, but Genzyme Homecare said this would not affect its position since it would be aimed at people for whom enzyme replacement was not suitable.