The decision to relax UK laws on cannabis is sending out the "wrong signal" to the rest of the world, according to a UN panel responsible for drugs issues.
Cannabis is to be downgraded this summer
Home Secretary David Blunkett's decision to downgrade cannabis from a Class B to a Class C drug may also damage British people's health.
According to the International Narcotics Control Board (INCB), relaxing the rules on cannabis may prompt an increase in its use.
The INCB's Nigerian president, Philip Emafo, said: "It is important that consensus prevails in international drug control.
"No government should take unilateral measures without considering the impact of its actions and ultimately the consequences for an entire system that took governments almost a century to establish."
He added that young people were confused over "mixed messages" about drugs.
It's quite worrying that we might end up in the next 10 or 20 years ... with our psychiatric hospitals filled with people who have problems with cannabis
"On one hand you are telling them not to go to clubs and use ecstasy because it is dangerous, but on the other hand you are not doing anything about cannabis," he argued.
In a 90-page report published on Wednesday, the INCB also suggested that the policy on cannabis could lead to increased cultivation of the drug destined for the UK and "other European countries".
Last September a conference in Nairobi heard the UK's decision would "undermine the efforts of governments of African countries to counter illicit cannabis cultivation, trafficking and abuse".
Mr Blunkett's initiative had led to "worldwide repercussions ... including confusion and widespread misunderstanding."
Critics of the government's stance say cannabis can cause cancers, heart disease and mental health problems.
Professor Ghodse, a former INCB president and in charge of studies into addictive behaviour at the University of London's St George's Hospital, said: "It's quite worrying that we might end up in the next 10 or 20 years ... with our psychiatric hospitals filled with people who have problems with cannabis.
"Recreational use of cannabis is something that any government and any community should think very seriously about."
Mr Blunkett is the home secretary
A recent study by the British Lung Foundation suggested that smoking three joints had the same impact as 20 cigarettes.
But the chief executive of drugs charity DrugScope, Roger Howard, argued that control of cannabis caused "disproportionately more harm to society than the harm caused by the substance itself".
He added: "The credibility of the INCB is thrown into doubt when its criticism of the UK Government's sensible proposal to re-classify cannabis is based on dubious science and misleading conclusions."
Professor Ghodse refused to be drawn on the specific issue of cautioning people found to be in possession of cannabis.
"The board does not dictate to the countries how to deal with penalties," he said.
"Penalties are a national issue providing they are not in violation of the conventions."
Although he welcomed Mr Blunkett's statement last July that all controlled drugs are to stay illegal.
The government will make the final moves towards downgrading the classification of cannabis later this year.
When that happens people found possessing small quantities of the drug will only be arrested in "exceptional" circumstances such as blowing smoke in a policeman's face or causing a disturbance.