The government in Spain's Catalonia region said it was investigating a clinic in Barcelona that is allegedly offering treatments to "cure" homosexuality.

The Policlinica Tibidabo in the Catalan capital is offering pills and psychiatric treatment to "convert" homosexuals, Spain's leading daily El Pais reported.

Many of those coming for treatment are followers of a particular religion who believe homosexuality is incompatible with their beliefs, it said.

"An investigation has been opened into this clinic," said an official for the regional government's health department.

"We do not consider homosexuality as an illness, far from it."

She said the clinic could face fines if the month-long probe concludes that such treatments are being carried out.

A gays and lesbian rights association in Catalonia, the CGL, hailed the decision of the regional authorities.

"It is totally unacceptable, in the 21st century, that health professionals are trying to treat homosexuality," CGL secretary general Antonio Guirado said in a statement.

"You cannot treat something that is not an illness."

Homosexuality was only legalised in Spain in 1979, shortly after the death of dictator Francisco Franco whose regime shipped off gays to institutions that some activists have likened to concentration camps.

The Socialist government of Prime Minister José Luis Rodriguez Zapatero has sought to promote gay rights as part of a strongly liberal social agenda.

In 2005 it passed a law to allow same-sex marriages, making Spain only the third member of the European Union, after Belgium and The Netherlands, to do so.