Berlin’s brothels are accustomed to all sorts of nocturnal visitors, but not, on the whole, to tax inspectors with clip-boards and stopwatches studying financial turnover.

The cash-strapped German capital this week dispatched inspectors into establishments such as Lust Land to drum up revenue from the city’s 7,000 sex workers. “Prostitution is a strong economic sector in the capital,” says Gerry Woop, spokesman for Berlin’s economic administration.

According to the German Institute for Economic Research, prostitution in Berlin has an annual turnover of €300 million (£203 million), making the sex industry more dynamic than domestic builders, painters and decorators.

Prostitution was legalised in 2002. This allowed brothel owners to provide more hygenic conditions of work and prostitutes became eligible for pensions and health insurance.

But the quid pro quo was that prostitutes should fill in annual tax returns. So far only a handful have been doing so.

Under the new rules, each prostitute will be required to pay €30 a day to the brothel owner as an advance payment on tax. The prostitute should have documentary proof for every day worked.

The association for prostitutes, HyDra, says the figure assumes unreasonably high earnings and should be reduced to €15. The service workers trade union, Verdi, is also backing the sex workers.

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