Detectives say the crime is rising steadily.

A seminar in Belfast on Saturday heard calls for people to be extra vigilant.

Jo Daykin, Northern Ireland's drugs strategy co-ordinator, said: "Watch your drink. Do not let your drink out of your site.

"Mind your friends. When people are going out for the night they should be very clear where they are going to be and that they are going to mind each other.

"Never leave a bar or a nightclub with a stranger and never accept a drink from a stranger."

Last month, a soldier was cleared of raping a student after spiking her drink with a date-rape drug at a nightclub in Edinburgh.

But Robert Lyttle, 26, from Co Down, Northern Ireland, was found guilty of another serious sexual offence against the woman.