Taking ecstasy is no more dangerous than riding a horse, according to the head of the Government's drug advisory body.
Writing in an academic journal, Professor David Nutt said taking ecstasy was no worse than the risks of "equasy", a term he invented to describe people's addiction to horse-riding.
Prof Nutt is the chairman of the Home Office's Advisory Council on the Misuse of Drugs which next week is likely to say that ecstasy should be downgraded to a Class B drug.
Campaigners last night called for him to resign, suggesting that he was on a "personal crusade" to decriminalise the drug.
Prof Nutt, who is an academic at Bristol University and Imperial College, London, wrote the article in the Journal of Psychopharmacology last month.
He said he wanted to compare the risks of horse-riding with the drug to open a debate about drug abuse and risk taking.
Prof Nutt told The Daily Telegraph: "The point was to get people to understand that drug harm can be equal to harms in other parts of life.
"There is not much difference between horse riding and ecstasy."
In the article, titled "Equasy: An overlooked addiction with implications for the current debate on drug harms", Prof Nutt wrote that "equasy", short for "Equine Addiction Syndrome", had caused 10 deaths and more than 100 road traffic accidents a year.
Through hunting, it also led to "gatherings of users that often are associated with these groups engaging in violent conduct.
"Dependence, as defined by the need to continue to use, has been accepted by the courts in divorce settlements," he wrote. "Based on these harms, it seems likely that the ACMD would recommend control under the MDAct perhaps as a class A drug given it appears more harmful than ecstasy."
He wrote that the risks of horse riding showed that society "does not adequately balance the relative risks of drugs against their harms".
He said: "Making riding illegal would completely prevent all these harms and would be, in practice, very easy to do.
"This attitude raises the critical question of why society tolerates – indeed encourages – certain forms of potentially harmful behaviour but not others, such as drug use."
There were plenty of other "risky activities such as base jumping, climbing, bungee jumping, hang-gliding, motorcycling" which were worse than which "many illicit drugs".
Campaigners said Prof Nutt's comments were ill-judged, coming ahead of the council on whether to downgrade the drug from A to B.
David Raynes, an executive councillor at the National Drug Prevention Alliance said: "Professor Nutt has made numerous unwise comments prejudging the ACMD review of Ecstasy. Is he on a personal crusade against the laws enacted by Parliament?
"He is entitled to his opinion, but if his personal view conflicts so very strongly with his public duties, it would be honourable to consider his position.
"If he does not, the Home Secretary should certainly do it for him."
The advisory council insisted that Prof Nutt was writing in the journal "in respect of his academic work and not as chair of the ACMD".
A spokesman said: "Prof Nutt's academic research does not prejudice the work that he conducts as chair of the ACMD."
There are 500,000 regular users and between 30 million and 60 million ecstasy pills in circulation in the UK.
If the advisory council recommends downgrading ecstasy next week, the Government can over-rule it - as happened with cannabis this year.
The council heard last year that deaths among ecstasy users had trebled from 10 to 30 a year over the past 15 years. The cost of pills had slumped from £15.50 to £2.30.
By Christopher Hope, Whitehall Editor
Last Updated: 8:02AM GMT 07 Feb 2009