A leading human rights organisation has urged revellers at this year's Pride Festival to exercise their rights if confronted by police with drug-testing kits.

Release, the law and human rights lobby group, believes gay men and women will be targeted during Pride.

It says they could be tested for drugs with Ion Track, the device which works by taking a swab from skin or clothes to detect minuscule traces of illegal substances.

Amber Marks, from Release, said: "Chief Inspector Lawrence Hobbs has assured Release that scans will not be used as a condition of entry and the police will only seek to scan those they already have reasonable grounds to search.

"There are concerns that the equipment has not always been used in this manner and may not always be used in strict accordance with the law."

Mrs Marks said Ion Track's use was covered by a grey area of the law because it is not defined whether it constitutes a search. This can only be carried out if an officer has reasonable grounds for suspicion.

She said: "This has yet to be determined by a UK court. There are doubts as to whether a positive reading on the device is capable of providing reasonable grounds of suspicion owing to the over-sensitivity of the equipment."

Release advised those asked to submit to an Ion Track test to co-operate with police but said it was everyone's right to demand a written copy of the officer's reasonable suspicions before undergoing a test.

Mrs Marks said: "If you are stopped and asked for your consent to a scan, ask whether they have reasonable grounds to search, detain or arrest you.

"If they say they don't, politely inform them that you do not wish to be detained by their enquiries, you value your civil liberties and understand you are under no obligation to assist them further."

Mrs Marks advised people who are tested to seek legal advice about bringing a civil court claim afterwards if they feel the tests was done without reasonable suspicions.

Release's intervention comes after members of the gay community raised fears over the use of Ion Track during Pride 2006. Organisers of the annual festival have given their permission to the police to carry out the swab tests.

But Richard Smith, of Bristol Road, Brighton, said: "This could ruin the atmosphere of what has always been a great event and I think even the police would admit a pretty much trouble-free day.

"Many people are going to feel uneasy and will be constantly looking over their shoulder."

Sussex Police attempted to reassure those going to Pride 2006 that the tests would only be carried out on those suspected of using drugs or those suspected of dealing.

A spokesman said: "Sussex Police wants everybody visiting Pride 2006 to have a safe and fun time. Our drugs testing policy isn't designed to catch anyone out or stitch them up."