By DAVID WILLETTS in Los Angeles Published: Today ACTOR Macaulay Culkin is the mystery dad of Michael Jackson's son Blanket, it was claimed last night. The Home Alone film star is said to have donated sperm to help Jacko create his seven-year-old offspring - real name Prince Michael II - with an unknown surrogate mum. The two celebrities became firm friends after Culkin, now 29, shot to fame as a child in the 1990 box office smash. Mystery ... rumours surround identity of Blanket's biological dad Rex Features He went on to star alongside Jackson in the video for his hit song Black or White in 1991, regularly visited the singer's Neverland ranch, and took the witness stand to defend him at his 2005 child molestation trial. One source said: "It is well known Jackson and Macaulay shared a unique bond. "Now rumours are spreading like wildfire that Macaulay, who Jackson nicknamed Mack, is actually Blanket's biological dad." Close pals say Jacko, who died aged 50 at his Los Angeles home in June, is not the natural dad of Blanket, his brother Michael, 12, or their 11-year-old sister Paris. Trusted Fevered speculation gripped the US after repeated claims that a "well-known Hollywood actor" donated the sperm for Blanket. Silence ... Macaulay Culkin Now sources close to Jackson say the Thriller star asked Culkin for the donation to help him complete his "perfect" family. The source added: "This isn't just chitter-chatter, even Culkin suspects he's Blanket's father. "So many names have been mentioned as prospective dads, and this is probably the wackiest yet. "But Jackson and Culkin were best friends. He was one of the few people Jackson really trusted and Mack never let him down. "Really, Jackson idolised him - that's why he asked Mack to donate sperm. "Deep down, I think he always wished Mack was his son. Creating Blanket was the next best thing." Culkin - godfather to Jacko's other two children - has told pals he will not comment in public out of loyalty to his late pal. Advertisement Helped by fertility experts and skin specialist Dr Arnie Klein, Jackson is said to have stockpiled the ingredients to genetically engineer three "perfect" kids. Klein is said to have given sperm for the elder two, while actor Mark Lester has claimed HE is Paris' dad. Yesterday Jacko's brother Marlon appeared to confirm the star did not conceive his children naturally. He said: "Those were Michael's kids - regardless of where they came from." Jackson will be buried on Thursday in a gold-plated coffin in the Hollywood Hills. Culkin was unavailable for comment.