Addressing his supporters at the end of his Socialist Party’s three-day congress in Valencia, Pedro Sanchez said the practice “enslaves” women.
The industry has flourished since its decriminalization in 1995 and estimates suggest that around 300,000 women work in the sex industry in Spain.
In 2016, the United Nations estimated Spain’s sex industry to be worth € 3.7 billion (£ 3.1 billion).
A 2009 survey found that one in three men had paid to have sex, but another report from the same year suggested that figure was almost 40%.
A 2011 UN study found Spain to be the third largest center of prostitution in the world, behind Thailand and Puerto Rico.
Sexual exploitation and pimping are illegal in Spain.
The industry is not regulated in Spain and there are no penalties in place for those who pay for sexual services, as long as they do not take place in a public space.
In his election manifesto in 2019, Mr Sanchez pledged to ban prostitution, which was seen to attract female voters.
He called this practice “one of the cruelest aspects of the feminization of poverty and one of the worst forms of violence against women”.
But, two years later, no law or bill has been tabled.
Public opinion on a ban is divided and in November 2018 there were nationwide protests to mark the United Nations International Day for the Elimination of Violence against Women.
Several protesters carried placards calling for a ban on the practice, and many were unhappy that the government had allowed the formation of a sex trade union.
The Organizacion de Trabadjaroras Sexuales (OTRAS) was given the green light in August 2018 and the group was formed in an attempt to create a safety net for workers.