Police believe incidents of date-rape are under-reported
Police posing as nightclub glass collectors found eight drinks had been spiked with "date-rape" drugs.
Officers in Essex tested 200 empty glasses, taken mainly from women, in a Chelmsford nightclub for traces of the drugs thought to be used in rapes.
Analysts set up screening devices in a room at the club to examine the drinks.
And one of the officers who took part in the operation said he was "amazed" by the results.
Sergeant Nigel Dermot, who led the survey, went into the club as an undercover nightclub employee with a second officer and two scientists.
The tests showed the presence of "date-rape" drugs in the glasses but further testing was needed to identify the exact illicit substances present.
Mr Dermot told BBC News Online he had been "amazed" by the results.
He said: "They are extraordinary...but it is far too early to draw any conclusions from them.
"There is no supportive evidence to suggest the community should be worried. It was a very simplistic trial."
He added that no-one whose glass had been tested had suffered any adverse effects from drugs.
Safe drinking practice:
Do not leave a drink unattended
If drinking from a bottle, keep your thumb over the top
Take your drink with you to the toilet
Do not accept drinks from strangers
Mr Dermot said one reason for the study was to raise the profile of spiked drinks over the Christmas period.
Essex Police said neither the town nor the nightclub in the study had a particular problem with these drugs.
They believe the results could be typical of clubs across the country.
There are no government figures on the number of drug rapes, but the Metropolitan Police receives about 100 allegations each year.
Attackers use the drug - which can be slipped into a drink - to disorientate the victim.
Senior officers there think the problem is under-reported.
"Date-rape" drugs such as Rohypnol and GHB - a liquid ecstasy - can cause victims to lose their memory. Traces of the drugs disappear very quickly and may not show up on tests.
It can also be hard to prove an accused attacker tricked the victim into taking the drugs.
However, the drugs are also used recreationally in nightclubs to increase the potency of drinks.