The US pharmaceuticals giant Merck & Co is accused of abusing its dominance of the market for an arthritis drug to block the NHS from using cheaper alternatives.

Competition authorities hit MSD, as Merck & Co is known outside the US, with formal charges that could lead to fines of tens of millions of pounds.

MSD allegedly attempted to prevent NHS doctors from using rival suppliers of a drug called infliximab, which it sells under the brand name Remicade, after its patent expired in 2015.

Infliximab is an antibody that suppresses part of the immune system and is used worldwide to treat a range of inflammatory conditions, including Crohn’s disease and psoriasis, as well as arthritis. About 100,000 NHS patients are treated with it at a cost of approximately £150m per year.

The drug is made by culturing genetically engineered cells. This complex process makes it difficult for MSD’s new rivals to produce versions identical to Remicade.

Instead they produce so-called biosimilar treatments which are close facsimilies. Doctors have been cautious about switching to biosimilar infliximab, however, fearing the effect on treatment of any tiny changes.

MSD allegedly exploited this caution by offering a discount on Remicade that would be withdrawn from buyers who tried alternatives. The Competition and Markets Authority (CMA) said it had provisionally found that the discount scheme was a breach of competition law, “likely to restrict competition from biosimilar versions of infliximab that were new to the market”.