Five police forces across England will carry out drug and alcohol tests on all women who claim to have been raped or assaulted over one weekend next month.

The study has been prompted by an explosion in claims of date rape linked to drugs such Rohypnol and GHB (gamma hydroxybutyrate), which make a victim drowsy, compliant and unable to remember what has happened to them.

Despite the claims, there has been no successful prosecution for the use of Rohypnol and some police officers believe the threat could be more imagined than real. They are backed by experts who say heavy consumption of alcohol is more likely to make women vulnerable to attack.

The study is being led by Derbyshire police and will include forces in Greater Manchester, Lancashire, Northumberland and London.

Detective Chief Superintendent Dave Gee, of Derbyshire police, said: ??The myth of Rohypnol has been generated by people talking about it all the time, not by any evidence.

??It has never been identified in any of the samples at the Weatherby forensic science laboratory, which does most of the testing for the north of England. There hasn??t been a single case successfully taken to court in the UK.

??Rohypnol is so notoriously difficult to get hold of, you are extremely unlikely to have it in your drink.??

Doctors also question whether drug rape actually exists. One forensic doctor said: ??We get situations where women admit to having had

20 units of alcohol in a week. We have to ask, ??Which of the 20 vodkas do you think was spiked????? Dr Cath White is clinical director of the St Mary??s centre in Manchester, one of seven sexual assault referral centres participating in the study. She insists alcohol consumption is the greatest risk factor for rape.

??There has been a huge increase in allegations of drug-facilitated rape. We are involved in this research to find out whether this is an urban myth or a real issue. No one really knows.

??There has not been a proven case of rape using Rohypnol, but if you pick up women??s magazines you would think it was widespread. We get the feeling there are drugs out there but I think the biggest problem is alcohol.??

During the trial, women alleging rape will give a urine sample that will be tested for traces of drugs and alcohol. Although urine samples are frequently taken from victims, they are not routinely analysed. The tests are expensive and are not carried out unless police are pressing ahead with an investigation.

Rape support workers blame the police for the absence of a Rohypnol conviction. Rohypnol leaves the bloodstream after about six hours and is not detectable in urine after 72 hours, which means tests must be taken immediately.

A recent police survey at a nightclub in Chelmsford, Essex, found eight drinks out of 200 were spiked.

Graham Rhodes, founder of the Roofie Foundation, a support group for victims of drug rape, said: ??Are we saying the 6,000 people who have called our helpline claiming to have been drug-raped are lying???

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