The British Government's decision to downgrade cannabis to a Class C drug has been criticised by the head of the United Nations anti-drugs department.

Antonio Maria Costa, executive director of the UN Office on Drugs and Crime (UNODC), said countries got the "drug problem they deserved" if they maintained inadequate policies.

In an unusual statement, he suggested cannabis was as harmful as cocaine and heroin - a stance which differs wildly from the British attitude of treating cannabis far less seriously than Class A substances.

Although he did not specifically name the UK, Mr Costa said at the Washington DC launch of the UNODC's 2006 World Drug Report: "Policy reversals leave young people confused as to just how dangerous cannabis is.

"With cannabis-related health damage increasing, it is fundamentally wrong for countries to make cannabis control dependent on which party is in government.

"The cannabis pandemic, like other challenges to public health, requires consensus, a consistent commitment across the political spectrum and by society at large."

Mr Costa suggested that cannabis was now "considerably more potent" than a few decades ago and that it was a "mistake" to dismiss it as a soft, relatively harmless drug.

"Today, the harmful characteristics of cannabis are no longer that different from those of other plant-based drugs such as cocaine and heroin," Mr Costa said.

The report estimated 162 million people used cannabis at least once in 2004, the equivalent of 4 per cent of the 15 to 64-year-old global population.

Mr Costa said: "After so many years of drug control experience, we now know that a coherent, long-term strategy can reduce drug supply, demand and trafficking.

"If this does not happen, it will be because some nations fail to take the drug issue sufficiently seriously and pursue inadequate policies."

David Blunkett downgraded cannabis from Class B to Class C in January 2004, meaning possession of the drug was normally no longer an arrestable offence.

In his speech, Mr Costa also repeated warnings about growing cocaine use, particularly in western Europe where demand was reaching "alarming levels".

He said: "I urge European Union governments not to ignore this peril.

"Too many professional, educated Europeans use cocaine, often denying their addiction, and drug abuse by celebrities is often presented uncritically by the media leaving young people confused and vulnerable."

His comments come less than two weeks after supermodel Kate Moss escaped prosecution for drug-taking, despite video evidence, because of a legal loophole.