A picture taken on October 28, 2012 shows Chelsea's Nigerian midfielder John Mikel Obi (R) talking with referee Mark Clattenburg (2nd R) as Spanish midfielder Juan Mata (L) looks on during the English Premier League football match between Chelsea and Manchester United at Stamford Bridge in London. London's Metropolitan Police have confirmed that they have received a complaint about allegedly racist comments made by a top flight referee during last weekend's match between Chelsea and Manchester United. Clattenburg is alleged to have used "inappropriate language", reportedly of a racist nature, towards Chelsea's Nigerian and Spanish players John Mikel Obi and Juan Mata at Stamford Bridge on October 28, 2012. AFP PHOTO/ADRIAN DENNIS RESTRICTED TO EDITORIAL USE. No use with unauthorized audio, video, data, fixture lists, club/league logos or 'live' services. Online in-match use limited to 45 images, no video emulation. No use in betting, games or single club/league/player publications. (Photo credit should read ADRIAN DENNIS/AFP via Getty Images)