Sources have told Sky News that the man being held is thought to be McCluskie's brother.
A murder investigation was launched after the discovery of the body and police are appealing for witnesses.
A Metropolitan Police spokesperson confirmed yesterday that a body had been found in Regents Canal near Broadway Market in East London and that enquiries were under way to establish the identity of the deceased.
Sources told have said that friends and relatives of the missing 29-year-old actress were "fearing the worst".
"At this early stage officers believe they know the identity of the victim but must await further forensic tests before formal identification can take place," The Independent quoted a police spokesperson as saying.
Over 100 people have assisted police in the search for the actress in the Bethnal Green area, while EastEnders stars Martine McCutcheon, Brooke Kinsella and Natalie Cassidy have also helped with the search on Twitter.
McCluskie's EastEnders character Kerry Skinner was a close friend of Zoe Slater in the soap, and the great niece of Ethel Skinner. She also dated Robbie Jackson, who proposed to her at one stage.
Anyone with information is asked to call police on 020 8345 3985 or Crimestoppers on 0800 555111.