According to the capital club, video footage from the recent meeting between the sides at the Bernabeu shows the 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 dismissed complaints made by both clubs, while Jose Mourinho's controverial post-match comments are being treated as a separate incident, with a decision to be made on Friday as to whether or not the Portuguese coach faces punishment.