Customers of sex clubs and brothels in Switzerland have been targeted by a blackmail campaign that threatens to reveal photos of them or risk being exposed. The anonymous threat includes a demand for 1,999 francs (£1,413) to be paid within five days, or risk having the indecent images leaked.

The money is being demanded in the form of Bitcoin currency and the threatening letter warns sex club clients that their blackmailers have been watching them. The Local reported that roughly 50 customers of the Globe sex club in Schwerzenbach, Zurich, are among those targeted.

The anonymous letter allegedly stated: "We know what you've done, who with, how long for and for what price. But you know that too! We hope that you haven't forgotten; if so we can refresh your memory with some photos."

Manager of the Globe, Tele Zurich Fritz Muller, told the Local that the letter is an "empty threat". He went on to add that he thought the threat was likely a hoax as photography is forbidden inside the club. Mobile phones are also not allowed. Local police are reportedly investigating the threat.

Prostitution is legal in Switzerland and the country is home to a number of sex clubs and brothels. A website dedicated to information about prostitution in Switzerland noted that the Free Movement of Persons Agreement allows workers from the European Union to work in Switzerland for up to 90 days a year, resulting in European prostitutes going to Switzerland to work for short periods of time.

The website notes that there are approximately 14,000 registered sex workers in Switzerland and at least one in five men between the ages of 20 to 65 have visited a sex worker at least once.