It is the oldest trick in the underage drinker's book – lacing a soft drink with something a little harder.

But police now have the answer to young people hiding alcohol in a soft drink – a testing strip which immediately detects it.

Officers in Leicester city centre will use the strips when they are investigating reports of underage drinking in parks, on street corners and in other public areas.

They will also carry the two-inch strips when policing under-18s nights at city centre clubs.

Pc Jon Webb, of city centre police's violence and disorder team, said: "Kids have got used to the way we work. They know we can spot them from 100 yards if they are carrying a bottle or can of alcohol, so they are putting alcohol into pop bottles, thinking they will get past us.

"We can't always smell the alcohol. Sometimes, if vodka is mixed with Coke, we can't always tell.

"The tests tell us immediately if a young person's drink contains alcohol or not. The strip changes colour if there is alcohol and we can then confiscate the can or bottle.

"If it really is a soft drink, we can hand it back because the strips are completely sterile."

Angela Downing, 34, who lives near Mandela Park, off Welford Road and Waterloo Way, said: "Underage drinking in the park gets worse in the summer, so we're expecting problems to escalate over the next few months.

"Some of the kids will sit there with bottles of lager, cider or vodka, but others are definitely trying to hide it in soft drinks."

A New Walk resident, who asked not to be named, said youngsters regularly gathered to drink in the historic walkway.

She said: "I like the idea of the police being able to tell immediately if someone is trying to put one over on them.

"You see them with what look like soft drinks, but their behaviour tells you otherwise."

Roy Rudham, chief executive of Leicestershire's City and County Neighbourhood Watch, said: "Drinking at that age, they can be a nuisance, but at the same time, we have to bear in mind that they can also be extremely vulnerable.

"They will try to hide their alcohol, but this test means the police will be able to tell when they are trying to trick them."

The strips could be used to test drinks carried by adults suspected of breaking a street-drinking ban.

However, police said most of those who drink in the street do not try to hide the fact that they have alcohol.