Uruguay's legalisation of marijuana violates an international drug control convention and fails to consider a negative health impact, a body set up to monitor compliance with the five-decade-old treaty said this evening.
The president of the International Narcotics Control Board, Raymond Yans, said the change would not protect young people.
He said it would have the "perverse effect of encouraging early experimentation" and lowering the age of first use.
Adding weight to the criticism of the move by Uruguay - the first country to take such a step - the UN anti-drugs office said it agreed with the INCB.
It said that states should work closely together to deal with the global drugs challenge.
"It is unfortunate that, at a time when the world is engaged in an ongoing discussion on the world drug problem, Uruguay has acted ahead of the special session of the UN General Assembly planned for 2016," David Dadge, spokesman for the UN Office on Drugs and Crime, said.
In an experiment that will be closely watched by other nations debating drug liberalisation, Uruguay became the first country to legalise the growing, selling and smoking of marijuana.
A government-sponsored bill approved in the Senate provides for regulation of the cultivation, distribution and consumption of marijuana and is aimed at wresting the business from criminals in the small South American nation.
But the Vienna-based INCB said the legislation contravenes the 1961 Single Convention on Narcotic Drugs, to which it said Uruguay is a party.
The convention requires states to limit the use of cannabis to medical and scientific purposes, due to its dependence-producing potential, Mr Yans said in a statement.
He was surprised that Uruguay's legislature and government "knowingly decided to break the universally agreed and internationally endorsed legal provisions of the treaty".
The INCB describes itself as an independent, quasi-judicial body charged with promoting and monitoring compliance with the three international drug control conventions.
It called on Uruguay "to engage with the board with a view to ensure that Uruguay continues to respect and implement the treaties to which it is a party".
But the statement did not say whether it envisages any further action on the issue.
Uruguay's attempt to quell drug trafficking is being followed closely in Latin America, where the legalisation of some narcotics is being increasingly seen by regional leaders as a possible way to end the violence spawned by the cocaine trade.
Rich countries debating legalisation of the drug are also watching the bill, which philanthropist George Soros has supported as an "experiment" that could provide an alternative to the failed US-led policies of the long "war on drugs".
Other countries have decriminalised marijuana possession andthe Netherlands allows its sale in coffee shops, but Uruguaywill be the first nation to legalise the whole chain fromgrowing the plant to buying and selling its leaves.
"Just as illicit drugs are everyone's shared responsibility, there is a need for each country to work closely together and to jointly agree on the way forward for dealing with this global challenge," Mr Dadge said.
Mr Yans said Uruguay's decision "fails to consider its negative impacts on health since scientific studies confirm that cannabis is an addictive substance with serious consequences for people's health".
"Cannabis is not only addictive but may also affect some fundamental brain functions, IQ potential, and academic and job performance and impair driving skills.
"Smoking cannabis is more carcinogenic than smoking tobacco," the INCB statement said.