The seven – believed to be aged from mid 20 to mid 50s – were semi-conscious when paramedics and police arrived at the house at Bowland Place, Moor Nook, at 6pm on Wednesday.

Police believe the group - who were supervising the party - had taken a controversial legal party drug called Gamma-Butyrolactone (GBL), which is used by revellers to achieve a 'party high'.

Five were taken to hospital by ambulance and two were admitted later.

The drug, which is banned in North America and some parts of Europe, is taken in a liquid form and is similar to the date rape drug GHB. In April, 21-year-old Hester Stewart from Brighton, died after taking the drug on a night out.

Neighbours in Bowland Place reported seeing the group being loaded into a row of ambulances.

One woman who did not wish to be named: "It was just beyond belief.

"There were at least four fast response (ambulance) cars there, all lined up and loading people in the back."

Police confirmed that the incident took place at a child's birthday party and that a number of parents, friends and relatives were taken to hospital.

Det Con John Billington said no children were injured or taken to hospital.

The seven people have all been released from hospital or discharged themselves, he added.

A statement released by Lancashire Police, said: "The Ambulance Service was called to an address in Ribbleton, Preston, where five adults required medical attention having taken some form of drug together with alcohol, rendering them semi-conscious.

"Although it is too early to be certain exactly what the drugs were, it is believed to be GBL, a legally available tranquilliser purchased via the Internet.

"All five were taken to RPH."

A Royal Preston Hospital spokesman confirmed a group did receive treatment having arrived at the hospital "under the influence".

When a reporter approached some of those believed to have been at the party, they refused to comment.

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