Politicians in Germany have called for the laws on prostitution to be tightened to protect women being forced into sexual slavery.

Prostitution was legalised in Germany in 2002, and 400,000 people are now registered as sex workers in the country.

Under the laws, prostitutes working in Germany have full access to health insurance, and declare their earnings as freelancers in their annual tax returns.

But the centre-right Christian Democrats (CDU) want the minimum legal age of sex workers to be raised from 18 to 21, as experts estimate that thousands have been trafficked into the country and forced to work in brothels against their will.

They also want those punishments introduced for men who use women forced into prostitution, and for compulsory health checks to be introduced for sex workers, to protect their health.

According to police, 50 to 90 per cent of the country's prostitutes have been forced into the work against their will, with many having been transported in from countries in eastern Europe to work in hundreds of brothels across the country.

With the legalization of prostitution, a series of enormous 'mega brothels' have sprung up in Germany, some able to accommodate hundreds of customers at a time, offering a flat rate for unlimited sex, and spreading over several floors.

The centre-left Social Democrats (SPD) rejected these suggestions on Friday as "counterproductive".

SPD deputy leader Carola Reimann said that the new rules may deter men coming forward with information on women who have been trafficked into the country, for fear of being prosecuted.

She also argued that if the proposals became law, women under 21 would be driven to work illegally without legal protection.

The Christian Social Union, the CDU's Bavarian sister party, attacked the SPD and the Green party for opposing attempts to more closely regulate prostitution.

"It was the [SPD and Greens'] prostitution law of 2002 that meant Germany became the brothel of Europe in the first place," CSU MP Max Straubinger told the Passauer Neue Presse on Friday.

The issue will be debated next week, at a meeting of the Federal Ministry for Family Affairs.

Experts are currently being consulted for the draught bill, which will come before the German parliament in the Autumn.