BANGKOK (AFP) - Thai police are checking dozens of Bangkok mega-brothels to see if they are illegally siphoning off groundwater for "soapy massages" offered to clients that may be contributing to the gradual sinking of the capital.

The crackdown was launched after a trafficking raid on the "Victoria Secret" massage parlour found underage sex workers, a ledger listing bribes to officials and evidence the venue was stealing groundwater to avoid paying pricey utility bills.

Environmental officials say the illegal tapping of water contributes to the sinking of Bangkok, a low-lying city built on the banks of the Chao Phraya river.

More than 40 "soapy massage" parlours - huge brothels with dozens of private bathrooms where customers receive sexual services - are being inspected for water theft.

"Today we will... examine the water quality of each room (that) we will go inside," Suwat Inthasit, deputy commander of Natural Resources and Enviroment Crime Suppression Division, told reporters before leading a raid on the "Embassy Entertain" massage parlour in a Bangkok district notorious for its smutty nightlife. "We suspect they have illegally used groundwater," he said.

Environmental officers can test the water to determine whether it is from a legitimate piped source or illegally taken from the ground.

Experts have warned that parts of Bangkok could be submerged by 2030, chiefly due to rising sea levels and the draining of groundwater in the capital's swampy soil.

The practice exploded during the city's rapid development several decades ago.

Efforts in recent years to regulate groundwater use have drastically slowed the city's sinking rate from a peak of around 10cm a year in the late 1970s.

But the massage parlours' breach of regulations has revealed yet another way in which notorious venues fail to be above board.

Although prostitution is technically illegal in Thailand, it is widely tolerated and openly practised, with gaudy brothels dominating some areas of the capital.

A system of bribes and loopholes keeps the lucrative industry afloat, with police largely turning a blind eye unless brothels are suspected of employing underage or trafficked sex workers.

Public officials are routinely implicated in kickback scandals allowing the sex industry to flourish, but police rarely face prosecution in a country where wealth and rank shield wrongdoers from the law.