As drugs minister for two years and more recently as defence secretary visiting the opium-producing region of Afghanistan, Bob Ainsworth says he saw how a policy of prohibition has failed to reduce the harms that drugs cause: in his words, "fuelling burglaries, gifting the trade to gangsters and increasing HIV infections".

Now, he argues, it is time to make all drugs available legally within a strict system of regulation - either prescribed by doctors or sold under licence like tobacco.

Many people find inconceivable the idea that you could pop to the High Street and buy some cannabis or ecstasy along with a packet of twenty and a bottle of scotch. The notion that a doctor might sign a script for pure cocaine or diamorphine might seem equally extraordinary.

But Bob Ainsworth's ideas reflect the situation that existed in Britain in the last century. Until 1916 you could buy cocaine and heroin over the counter in Harrods. Shop assistants might have suggested "Ryno's Hay Fever and Catarrh Remedy" (basically pure cocaine) "for when the nose is stuffed up, red and sore". And what better way to support the boys at the front during World War I than Harrods gift packs containing morphine and cocaine?

Until the mid-sixties in Britain, doctors could and did prescribe heroin and cocaine to patients. Records confirm that in 1962 one London doctor prescribed more than 600,000 heroin tablets to hundreds of users.

The patient list of psychiatrist Lady Isabella Frankau reads like a Who's Who of sixties bohemian cool. Poets, actors, musicians, writers and refugees from the strict drug laws in the US and Canada knew that Lady F would not ask too many questions and, if you were a bit short of readies, might even waive her consultancy fee. American jazz trumpeter Chet Baker turned up at her door and later recalled how "she simply asked my name, my address and how much cocaine and heroin I wanted per day".

So Mr Ainsworth seems to be essentially calling for a return to a situation that was once described as "the British System" of narcotics control - regarding drug use as a health rather than a criminal matter.

The theory is that if you regulate the supply of drugs, so that they are available legally, you take the trade away from criminal gangs. Instead of buying heavily-adulterated and dangerous heroin from a street dealer, a user could obtain quality-controlled morphine from a GP - and be encouraged to get treatment and support to overcome addiction.

Mr Ainsworth is not the first drugs minister to change their tune on prohibition once leaving office. His predecessor in the job Mo Mowlam wrote an article in the Guardian in 2002 in which she said pretty much the same thing, as British troops fought the Taliban in Afghanistan.

"May I suggest that rather than bombing civilians in various Muslim countries, the United States and Britain begin to take a more intelligent approach to the international drugs trade: namely, to legalise it. For by doing this, not only will we help solve one of the major problems facing the world today, the unregulated growth of drugs trafficking, but it would also further isolate the terrorists."

Mr Ainsworth knows that public attitudes and the political weather are against him - so, rather than simply urging that ministers end the system of prohibition, he wants an impact assessment to be conducted on the Misuse of Drugs Act 1971 - the legislation which introduced drug classification in the UK:

"I call on those on all sides of the debate to support an independent, evidence-based review, exploring all policy options, including: further resourcing the war on drugs, decriminalising the possession of drugs, and legally regulating their production and supply."

Last week, in the foreword to a new UK drugs strategy, the home secretary said: "This Government does not believe that liberalisation and legalisation are the answer. Decriminalisation fails to recognise the complexity of the problem." Indeed, as reported at this blog , the government only considered two options in respect of its drugs strategy: doing nothing or implementing a prohibition-based strategy.

In my piece on BBC Breakfast this morning, Minister for Crime Prevention James Brokenshire said: "We don't think legalisation is the answer because it ignores why people actually get addicted to drugs in the first place. Those are a number of very complicated factors: some of them inter-generational, some of them relating to issues like homelessness or mental health."

However, Mr Ainsworth joins a growing number of public figures in Britain and around the world who are arguing for a rethink on global drugs strategy, among them chairman of the Bar Council Nicholas Green and former head of the Royal College of Physicians Sir Ian Gilmore.

Mr Ainsworth agrees that he has come a long way since his time as drugs minister. Then, in a debate in 2002, he warned Parliament that legalising drugs could see a rise in some crimes and that availability would inevitably increase with the risk that children as young as 10 could start using heroin and cocaine.

One MP on the opposite benches took a different view. "I ask the Government not to return to retribution and war on drugs. That has been tried, and we all know that it does not work," the member said. The rising star of the Conservative back benches had recently urged ministers to engage with the United Nations in considering legalisation and regulation of drugs. The MP's name was David Cameron.