Drug companies could face investigation following allegations that they are inflating the cost of drugs sold to the NHS.

The Serious Fraud Office is considering launching an inquiry into claims that pharmacists and drug companies may be secretly joining forces to overcharge the NHS by millions of pounds.

An undercover investigation by the Telegraph found that the prices of more than 20,000 drugs could have been artificially inflated.

According to the newspaper, representatives of some companies agreed to invoice chemists for drugs at up to double their actual cost.

Chemists would then send inflated invoices to the NHS, allowing them to pocket the difference. The taxpayer is thought to have lost millions of pounds as a result.

Labour MP Emily Thornberry, the shadow attorney general, has written to the SFO asking it to investigate the claims.

In his reply, SFO director David Green said: “The SFO is working closely with NHS Protect [which tackles crime in the health service] and the allegations that have been made are the subject of careful consideration.”

Meanwhile, the Office of Fair Trading said it is considering an investigation after the price of a women’s prescription medicine rose from £26 a pack to £395.

Intrinsa, a testosterone patch normally prescribed to menopausal women with low libido, was previously covered by a pricing cap for licensed medicines. But its licence was withdrawn voluntarily last year by initial producer Warner Chilcott.

The drug was reclassified as an unlicensed “special”, meaning it was no longer subject to a 29 per cent profit cap agreed by the Pharmaceutical Price Regulation Scheme. When HFA Healthcare bought the final batch, it then was free to set the price of the medicine.

As a result of the price rise, the annual cost to the taxpayer of supplying the drug to the NHS is expected to rise from around £500,000 to more than £7m.

HFA is not accused of doing anything illegal, and it is not one of the companies being examined by the SFO. But the OFT began reviewing the case after being contacted by a leading competition lawyer, who raised concerns about the huge price increase.

GP Bill Beeby, who chairs the British Medical Association prescribing committee, called the price hike “outrageous”.

HFA Healthcare general manager Faisal Janjua said: “Our only commercial option, as the licence had been cancelled, was to supply as an unlicensed product. Any business has to recoup significant capital outlay so the price was modified.

“Another reason for the higher price is that we intend over time to re-licence the product. There are significant costs to re-licence.”