A German tourist was the latest visitor to feel the wrath of the sex workers when he was clubbed around the head with a rock and had his money and valuables stolen in the party town of Pattaya, south of Bangkok.

Lutz Mohler told police he had been attacked when he refused the advances of a group of the transgender sex workers, adding: "When I refused, they hit me with a rock and rode off on a motorbike.”

His attackers were arrested just hours later after leaving their victim in hospital with a broken nose.

It was just the latest in a string of vicious assaults that Thai police are trying to quell as the ladyboys increasingly target Westerners visiting the south-east Asian country.

In December a gang of four were arrested in the town of Pattaya after Naami Keyghobadi, from Leeds, plunged from a nightclub's fourth-floor window.

It is alleged the 28-year-old had been in an argument with the 'ladyboys' over a payment only minutes earlier.

Thai police have now admitted they are struggling to keep in check a growing number of aggressive sex workers attacking foreign tourists.

Colonel Suktad Poompanmuang said: "This is of great concern to us.

"The ladyboys are not only aggressively attacking tourists to rob them, they are also severely harassing them for sex. It makes the streets very dangerous late at night.

"There’s a 1000 baht (£20) fine for sexual harassment, with possible imprisonment for causing serious injury, but that hasn’t cut down the problems.

"We can only advise tourists to be very careful about who they speak to late at night and avoid being out and about alone at that time."

Police confirmed several other attacks on tourists within hours of the vicious assault on Mr Mohler.

A Chinese man, 26, was earlier robbed of jewellery as a gang of ladyboys harassed him for sex.

While an Indian tourist was targeted for a neck chain only 30 minutes later.