We live in a country where the prime minister refuses to say whether he has ever taken drugs. As Mr Cameron once ticked a multitude of boxes on the drug squad's offender profile – rich, single, young man working in target-rich environment of a London TV company's PR department – I can guess the cause of his embarrassment. The leader of the opposition, meanwhile, has appointed as his spin doctor Tom Baldwin of the Times, who has never responded to uncorroborated allegations in a book by the Conservative peer Lord Ashcroft that he could well have had a fondness for cocaine when he was a hell-raising reporter about town.

What they and many others at the top of politics may or may not have swallowed, snorted or injected would be no concern of yours or mine, if they allowed an intelligent argument about drugs policy.

Indeed, if they had swallowed, snorted or injected without noticeable ill-effect, that argument ought to be easier. But honest argument is not possible in Britain as the contemptuous and contemptible treatment of Bob Ainsworth showed. Perhaps inspired by Ed Miliband's cry that he led a "new generation that understands the call of change", Ainsworth called for a change to the drugs trade. He wanted to take it out of the hands of criminals, who do not care how dangerous their wares are as long as they get their money, and hand it to accountable public servants. If Britain regulated production and supply, he argued, chemists and doctors could prescribe hard drugs.

Drug takers would escape the death and sickness adulterated drugs bring. The public would find some escape from junkies stealing to fund their addiction, and from crime syndicates, whose wealth makes them ever more able to corrupt the criminal justice system.

Ainsworth was a Labour Home Office minister, responsible for drugs policy, and for that reason alone his views were worth listening to. He had also served in the Ministry of Defence and seen the impossibility of fighting both a war on terror and war on drugs in Afghanistan. His realisation that conventional wisdom was creating the monsters he was in politics to oppose concentrated his thinking.

He told me how ashamed he became as a minister when he saw Britain, acting at the behest of America, condemning Portugal for liberalising drugs laws, a policy that has seen striking falls in HIV infection in the slums of Lisbon. He pointed me to the disaster current orthodoxy has brought to Mexico, which is in a kind of civil war between the cartels and the government, and Jamaica, where drug-funded corruption is creating a failed state.

He had learned lessons from the political failures he had witnessed, which is more than I can say for his opponents. When David Cameron stood for the Tory leader he favoured decriminalisation and allowing doctors to prescribe heroin.

His forward-thinking did not survive contact with office. At his behest, a succession of Conservative MPs denounced Ainsworth for advocating the "devastation of communities", condemnations they might more justly have directed against themselves considering the gangsterism and misery their policies have created. I will say this for the Tories, however: at least they stood up in public view. Labour showed that under Ed Miliband it was reverting to the sneak attacks of Charlie Whelan and Damian McBride.

A Labour spinner – I don't know if it was Baldwin or one of his underlings – could not confine himself/herself to saying that Miliband disagreed with Ainsworth. Instead, the unelected and cowardly "Labour source" went for an elected and brave Labour politician anonymously. He/she told the lobby journalists who maintain the poison-pen culture of the aptly named "Westminster village" that Ainsworth was an "extremely irresponsible" man. "I don't know what he was thinking."

In that moment, the "source" crystallised a doubt about Miliband's "new generation" that had been nagging at the back of my mind. Your ideas and principles matter in politics, not your age. The "new generation" on both the Labour and the Tory side is not bringing new thinking but repeating the worst failings of the old. When confronted with a difficult social problem, they retreat into know-nothing denial.

They display their bad faith by damning two distinct reforms and treating them as one. Liberalisation usually means following different policies for different illegal drugs, not punishing users for possession and decriminalising cannabis use. In theory, you can disapprove of liberalisation and want to keep illegal drugs illegal, while still believing that it is better to cut off the flow of money to crime syndicates by allowing doctors to prescribe hard drugs to addicts. This was the policy known throughout the world as "the British system". From the 1920s, doctors provided an alternative source of drugs to addicts, who could not break with their dealers. With the collaboration of the medical profession, I am sorry to say, the Home Office ended the system in the 1960s and the result is the crisis we see around us.

In Switzerland, Germany and Canada, doctors are reviving the British system. With a minimum of fuss, Britain is too. Professor John Strang of King's College, London, has run pilot projects, which Ainsworth set up – "very quietly because Downing Street would have gone wild if it knew what I was doing". The worst addicts were given heroin and their behaviour was compared with the behaviour of a control group taking methadone, the medical profession's preferred and often inadequate substitute. Unsurprisingly, methadone users also bought street heroin when they were out of the sight of doctors. Urine tests showed, however, that those given heroin gave up on the criminals' drugs. Several told the professor that taking drugs in a supervised clinic meant that they stopped injecting in the groin. They were building the strength to break free from the circle they shot up with.

Even if the coalition found the will to expand the pilots nationally, you can guarantee that they would back off as soon as the press started shouting that taxpayers were subsidising junkies' pleasures (although "pleasure" is the last thing addicts find in drugs). The Home Office's cowardice is already evident. Its latest drug policy paper devotes just one line to the most effective anti-crime strategy it has.

People often say we have the prohibition of drugs in Britain. But illegal drugs are not prohibited, they are everywhere. What we have is a prohibition of political debate on what to do with them and that is the greatest drug crime of all.