According to the capital club, video footage from the recent meeting between the sides at the Bernabeu shows the Barca midfielder issuing a slur towards the Brazilian left-back
The footage, which can be seen at the 1:50 mark of this video at RealMadrid.com, shows Busquets partially covering his mouth while speaking to Marcelo.
The video's captioning seems to claim that Busquets is saying, "Mono, mono," which translates from Spanish into English as, "Monkey, monkey."
Barcelona declined to give comment to Goal.com regarding the allegations contained in the video.
Busquets was one of six players reported by Madrid to European governing body Uefa for 'unsporting conduct' following the recent meeting of the two sides.
Uefa have reportedly dismissed Real Madrid's complaints but are conducting their own disciplinary procedures against the two teams.