The International Narcotics Control Board (INCB) says it is concerned by the approval by Uruguayan MPs of a bill which would legalise marijuana.
The INCB says the law would "be in complete contravention to the provisions of the international drug treaties to which Uruguay is party".
Under the new law, the state would assume control of growing and selling cannabis to registered users.
The bill still needs to be passed by Uruguay's senate before becoming law.
'Serious consequences'
The INCB is an independent body of experts established by the United Nations to monitor countries' compliance with international drug treaties.
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Uruguay draft marijuana law
- State would control the production, distribution, commerce and consumption of marijuana
- Consumers have to be over 18 and registered
- Marijuana to be sold at licensed pharmacies
- Sales are limited to 40g (1.4oz) per month
- Registered users can grow up to six plants at home
In a statement released just hours after the bill was passed in Uruguay's House of Representatives, the INCB said that such a law would be in "complete contravention" of the
Single Convention on Narcotic Drugs of 1961, which bans the sale of cannabis for non-medical use.
It says that, if adopted, it "might have serious consequences for the health and welfare of he population and for the prevention of cannabis abuse among the youth".
The Uruguayan government argues that by bringing the sale of cannabis under state control, it will remove profits from drug dealers and divert users from harder drugs.
Political tussle
The bill was approved by 50 of the 96 MPs present in the House of Representatives following a fierce 13-hour debate in the capital, Montevideo.
MPs debated for 13 hours before the bill was passed by 50 votes to 46
The supporters of the measure argued that the fight against drugs and drug trafficking had failed, and the country needed "new alternatives".
"The regulation is not to promote consumption; consumption already exists," said Sebastian Sabini of the governing centre-left Frente Amplio (Broad Front) coalition, which has a majority of one in the lower house.
Marijuana use has reportedly doubled in Uruguay over the past years. An estimated 22 tonnes of marijuana are being sold in the country annually, according to Uruguay's National Drugs Committee.
But Gerardo Amarilla of the opposition National Party said the government was "playing with fire" given the health risks he said were linked to marijuana use.
The bill will now go to the Senate, where it is expected to be voted on in October. As the left-wing government has a majority in the Senate, the bill is likely to be approved and could become law before the end of the year.
However, opposition politicians have said that even if the law makes it through the senate, they will launch a petition to have it overturned.
A survey carried out before the vote by polling organisation Cifra suggested 63% of Uruguayans opposed the bill.
Papal opposition
The INCB said it was interested in "maintaining a dialogue with the Government of Uruguay on this issue".
It also expressed its regret that the government "refused to receive an INCB mission before the draft law was submitted to Parliament for deliberation".
The vote comes amid fierce debate about drug legalisation in Latin America.
A group of former presidents and influential social figures, including Brazil's Fernando Henrique Cardoso, Mexico's Ernesto Zedillo and Colombian ex-leader Cesar Gaviria, have called for marijuana to be legalised and regulated.
But only last week, Pope Francis criticised drug legalisation plans during a visit to Brazil.
Speaking at the inauguration of a clinic for drug addicts in Rio de Janeiro, he said it was "necessary to tackle the problems which are at the root of drug abuse, promoting more justice, educating the youth with the values that live in society, standing by those who face hardship and giving them hope for the future".
I have yet to meet someone in my life who doesn't agree that Marijuana is either less harmful, or "just as harmful" as Alcohol, yet the United Nations is freaking out over a country using it's own DEMOCRATIC fukkING PROCESS to legalize weed.
This is spot on.
I'm only 20) and get my own shyt I'm not going to smoke anymore. I have a few jobs lined through connections made by me and my dad but I need clean piss. 
