Pub and club revellers face a drugs test as soon as they enter the premises. Anyone going into a bar, whether they arouse suspicion or not, will be asked to take a swab test, which highlights any drug use.

The scheme is being run by police in south Staffordshire and will initially cover the towns of Cannock and Stafford.

Police have warned that anyone refusing will automatically arouse suspicion and have told establishments that do not co-operate that it will be held against them when their licences come up for renewal.

Liberty, the civil rights pressure group, said it was "deeply worried" and accused the police of operating "by coercion rather than by consent".

Chief Supt Nick Lowe, division commander, said: "The beauty of it is that it is so quick. It will allow us to test hundreds of people in a very, very short amount of time. A swab will be placed on the hand and will show up green, amber or red, depending on if there are drugs in the person's system.

"If it shows red, which means definite contact with drugs, the police can intimate their powers under the Misuse of Drugs Act to stop and search the person, and then arrest them if necessary.

"If it is green or amber no action will be taken. If someone refuses, then it is a tick in the first box of suspicion. Police officers are present and it may be that further questions will be asked."

The equipment used is a £40,000 computer the size of a briefcase, funded by the Communities against Drugs Fund. A swab on the back of the hand, which is then fed into the computer, will test for ecstasy, cannabis, heroin, cocaine, amphetamines and also rohipnol, the so-called date rape drug.

The test with results takes about eight seconds and does not provide officers with a DNA sample. The swabs are thrown away immediately.

Mr Lowe said: "We have clear evidence that a large volume of crime is drugs-related, whether it be for violence, vehicle crime or anti-social behaviour.

"The tests appeal to the general majority of the public who want to use drug-free premises. Most people are happy to do it."

Because there is only one computer, the police will also be operating with dummy ones in other premises.

Gareth Crossman, a Liberty spokesman, said: "This is an extremely questionable use of police powers. The police cannot force someone who is not under arrest to take a drug test but they are implying they can.

"To then use a perfectly legitimate refusal to comply as part of the justification for suspicion is an abuse of policing powers."