Sex work is tolerated in Spain -- neither illegal nor regulated -- but Sanchez came to power in June with a strongly feminist agenda promising to fight the exploitation of women. 

"Prostitution in Spain isn't legal and this government won't support any organisation that includes this illicit activity," he tweeted from Colombia, where he stopped off on a Latin America tour.

He added the labour ministry had launched proceedings to challenge in court the existence of the "Organisation of Sex Workers", which promises to defend the rights of people in the industry. 

News that the union has been registered by the ministry's labour directorate general has embarrassed Sanchez.

Not only was it approved, but it was published in the official state gazette (BOE) on August 4.

The BOE said the union has its headquarters in Barcelona and has a nationwide remit.

In a statement, Spain's labour ministry said the government "cannot allow for prostitution to be validated under the subterfuge of a 'union' of sex workers".

Labour Minister Magdalena Valerio told reporters she was shocked by the fact such a union had been approved.

"As a minister and member of a feminist government, I would never have given the OK for this to be published in the official state gazette," she said.

Valerio added this has been one of "the biggest" upsets in her life.

Prostitution is legal and regulated in several countries in Europe, including the Netherlands, Switzerland, Austria and Germany.