The green signs painted on the street – showing a woman posing against a street light – aim to combat the problem of prostitutes looking for customers outside the ‘tolerance zone’ established by the city in the district of Kleinbasel.

Around 800 women work in the sex trade in Basel, the majority in salons or bars.

Some 30-50 work on the street in the Kleinbasel tolerance zone, but many do not seem to know the rules and stray from the designated area.

In a statement on Monday, Basel’s security and justice department said there was a high turnover of streetwalkers in the area, partly due to the influx of prostitutes arriving from Eastern Europe,  “which sometimes makes it difficult to convey the legal rules”.

In the past year police have caught 120 women touting for business outside the designated zone, said the statement.

The situation has angered residents. Six months ago a petition called for Basel police to take tougher action, reported Blick.

The new markings will clearly show prostitutes where the tolerance zone begins and ends, making violations “easier for the police to prosecute”, said the authorities.

Prostitution is legal in Switzerland though controlled by regulations, including city bylaws specifying strict zones for streetwalking.

The country’s initiatives on the issue include opening ‘sex boxes’ in Zurich where prostitutes may ply their trade in relative safety away from the city centre.

Despite the challenges in Basel, a ban on prostitution is not the answer since it would be more difficult to control if illegal, authorities told the Basler Zeitung on Tuesday.