Formal complaints cannot be anonymous and have to fulfil some requirements according to Act 23/2015, July 21st, on the Labour and Social Security Inspectorate and Royal Decree 928/1998, May 14th, on the Regulation on Sanctions Procedure and Social Security liability procedure.
Information on how to file complaints
Communication through the Labour Fraud Mailbox
Communications via the Labour Fraud Mailbox are anonymous and are used by the Regional Inspectorate to plan their investigation actions.
Tax administration (AEAT)
Website: https://www.agenciatributaria.gob.es/AEAT.sede/tramitacion/ZZ06.shtml (Information about the different types of complaint and procedures on the Tax Administration website)
Website available in English: https://www.agenciatributaria.gob.es/AEAT.sede/en_gb/tramitacion/ZZ06.shtml
Information and assistance: https://www.agenciatributaria.gob.es/AEAT.sede/en_gb/Ayuda/ZZ06.shtml
Complaints can be filled in person at the Police Forces offices, online, by telephone.
National Police information: https://www.policia.es/denunweb/denuncias.html
Guardia Civil information: http://www.guardiacivil.es/es/servicios/denuncias/index.html