The Vatican's objection to "harm reduction" strategies, such as needle exchange schemes, has ignited a fierce debate between the US and the EU over how drugs should be tackled.
A new UN declaration of intent is due to be signed in Vienna on 11 March.
However, there are major disagreements between member countries over whether a commitment to "harm reduction" should be included in the document, which is published every 10 years.
Now the Vatican has issued a statement that claims that using drugs is "anti-life" and "so-called harm reduction leads to liberalisation of the use of drugs".
The Vatican's last-minute intervention appears to have led to Italy withdrawing from the EU consensus on the issue and thrown the talks over the declaration into confusion.
In 1998, the declaration of intent was "a drug-free world - we can do it", which critics claimed was unrealistic and did not address the complex nature of drug treatment.
In favour of including support for a harm reduction clause are most EU countries, Brazil and other Latin American countries, Australia and New Zealand. They argue that some commitment to tackling HIV and addiction through needle exchange programmes and methadone and other drugs should be included.
Opposed to this are the US, Russia and Japan.
The US position has been that such inclusion sends the wrong message, although there have been indications a more liberal policy might be adopted under Barack Obama.
"By making a statement against harm reduction, the Vatican has indicated that its moral objection to drug use is more important than its commitment to the sanctity of life," said Release, the UK-based drugs and legal advice charity.
"If the Vatican is allowed to influence the UN to adopt a naïve and ineffective drug policy, it will needlessly lead to the increased spread of blood-borne viruses and the death of thousands more people from HIV/Aids."
Release argues that drug treatment programmes are vital for people suffering from HIV/Aids and that not to accept this will put their lives at risk. "Needle and syringe exchange programmes have significantly contributed to the reduction of HIV transmission among people who use drugs," it said.
A spokesman for the UN Office on Drugs and Crime (UNODC) confirmed last night that "there are still differences of opinion" over the statement.
Other drug charities and foundations have also been critical of the Vatican's move. Mike Trace, chairman of the International Drug Policy Consortium, said: "Progressive governments, including the UK, are now faced with the dilemma of trying to rescue reasonable support for harm reduction or not sign the declaration."
Antonio Costa, the head of UNODC, has claimed harm reduction has been appropriated by "a vocal minority". He said in a speech last November: "This kind of harm reduction can prevent the spread of blood-borne diseases. But it does not solve the underlying problem, and can even perpetuate drug use."
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