Zapatero had made a campaign pledge on the subject

His government will draft a law to put gay unions and marriage on an equal footing, he told Spain's Telecinco television channel on Thursday.

He said same-sex marriage was a characteristic of a "modern and tolerant society".

The conservative outgoing government of the Popular Party had rejected several calls to allow same-sex marriages.

Mr Zapatero swept to a surprise victory in general elections held on Sunday.

Legalising gay unions was one of his campaign pledges.

In the interview he did not set a time-frame for the change.