Legalising cannabis poses a "grave danger to public health and well-being", the UN has warned.

Moves to legalise marijuana in Uruguay and US states of Colorado and Washington were branded "misguided initiatives" by the head of the the International Narcotics Control Board (INCB), the UN body for enforcing international drug treaties.

Car accidents involving drug drivers testing positive for cannabis and cannabis-related treatment for teenagers have all increased in Colorodo since a commercial medical cannabis programme was "poorly implemented", the INCB claimed.

In an annual report, INCB president Raymond Yans said: " Drug traffickers will choose the path of least resistance, so, it is essential that global efforts to tackle the drug problem are unified.

"INCB is concerned about some initiatives aimed at the legalisation of the non-medical and non-scientific use of cannabis.

"Such initiatives, if pursued, would pose a grave danger to public health and well-being, the very things the states, in designing the conventions, intended to protect.

"INCB looks forward to maintaining an ongoing dialogue with all countries, including those where such misguided initiatives are being pursued, with a view to ensuring the full implementation of the conventions and protecting public health."

The warning follows a vote by Uruguay's parliament in December to approve a bill to legalise and regulate the sale and production of marijuana.

Meanwhile, licensed suppliers have been able to sell cannabis to adults aged over 21 in Colorado since January, while this is due to be repeated in Washington state this summer.

In the report, the INCB attempts to counter-attack some of the arguments put forward in favour of legalising the drug.

Supporters of more liberal drug laws argue that enforcement expenses resulting from the current international drug control regime, are the source of most costs.

However, the INCB said government revenue from the legal sale of alcohol and tobacco is less than the economic and health costs of their abuse, while there might be increased law enforcement costs due to higher crime rates occurring under more permissive laws.

"In many countries, alcohol, not drugs under international control, is responsible for far more arrests than... illegal drugs combined," it added.

Slapping down arguments that criminals will be deprived of earnings if drugs were legalised, the UN said organisations may enter the illicit markets, citing figures for the UK showing nine to 20% of the domestic cigarette market now consists of smuggled cigarettes.

"One can also imagine states having to bear regulation costs of such alternative drug regimes," the report said. "Costs of regulation include, among other things, monitoring and controlling cultivation, production, manufacturing and distribution, as well as monitoring use, and its impact."

It added: "If currently controlled substances were regulated as alcohol is in many countries, more people would use them and become addicted, resulting in more adverse consequences."