Sex workers in Hungary now become significant contributors to the country's legal economy as officials said Monday it would allow prostitutes to apply for a working permit.

The effort, which allows prostitutes to issue receipts to their customers and make social security contributions, in turn allows the approximately 20,000 prostitutes to generate for the Hungarian government as much as an estimated $1 billion annual revenue.

Agnes Foldi, head of the Hungarian Prostitutes' Interest Protection Association, said around 20 prostitutes have already been issued permits.

Describing Hungary's sex industry as "one of the leading sections of the shadow economy," Hungary's tax authority APEH's spokesperson Agnes Bakonyi said, "with this project, APEH is trying to help a group of professionals, in what is called the world's oldest profession, who have never paid taxes in their life,".

This development came amid flaks from human rights groups on Hungary's legalizing prostitution under certain conditions since 1999.

Janice Raymond of the U.S.-based Coalition Against Trafficking in Women said Hungary is violating its international treaty obligations under the U.N. convention by issuing such entrepreneurial permits to prostitutes, the Associated Press reported.

Prostitution is legal in Hungary but there are legal restrictions or tolerance zones, such as away from schools and churches, as to where a significant amount of constitution can legally occur. However, no zones were seen to be in effect in 1995, said a U.S. Department of State 2005 Country Report on Human Rights Practices.

Prior to this, police regularly fined prostitutes for offering their services, and often targeted them for physical and verbal abuse.

An estimated 7,000 to 9,000 prostitutes work full-time in Hungary with numbers rising in hte summer tourist season.