Sir Ian has recently stepped down as president of the Royal College of Physicians, and in a valedictory message to colleagues, he called for laws to be “reconsidered with a view to decriminalising illicit drugs use”. He said: “This could drastically reduce crime and improve health.”

Sir Ian said he agreed with the argument put forward by Nicholas Green QC, the chairman of the Bar Council of England and Wales, who said last month that it was “rational” to consider “decriminalising personal drug use”.

Sir Ian also said he was persuaded by a recent article in the British Medical Journal, which argued that the prohibition of drugs had been “counterproductive”, made many public health problems worse, and stimulated organised crime and terrorism.

Sir Ian said that banning drugs had harmed society. “There’s a lot of evidence that the total prohibition of drugs, making them totally illicit and unavailable, has not been successful at reducing not only the health burden, but also the impact on crime,” he said.

“I’m trying to take a fresh look, as many people have done. There is a strong case for a different approach.”

There should be a “regulatory framework around illicit drugs, rather than a blanket prohibition”.

Evidence suggested that state regulation of drug use “doesn’t increase the number of drug users,” he said.

Regulating drug use would mean “helping people with addiction problems, rather than putting them in prison”.

He also suggested that regulating drug use would save money on policing and on international efforts to reduce the cultivation of narcotics. “It’s more cost effective to try to treat people with drug problems than to close down poppy fields in disparate countries.”

Danny Kushlick of Transform, a drug reform campaign group, said Sir Ian’s statement was “a nail in the coffin” of the current drug laws.

Keith Vaz, chairman of the House of Commons home affairs select committee, said the legalisation of drugs “would simply create the mistaken impression that these substances are not harmful, when in fact this is far from the truth”.