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Culkin, the Home Alone star, is said to have donated sperm for Jackson's use out of friendship. Details of Blanket's mother remain under wraps but a rumour that she is a Mexican nurse living in southern California recently emerged.
Culkin, 29, appeared in a video for the hit song Black Or White in 1991 and was a regular at the singer's Neverland ranch.
A 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."
The rumour has been fuelled by repeated claims that a "well-known Hollywood actor" donated sperm that was used to father Blanket. A source said: "This isn't just chitter-chatter, even Culkin suspects he's Blanket's father... Jackson and Culkin were best friends. He was one of the few people Jackson really trusted and Mack never let him down."
The source pointed to Jackson's deep affection for Culkin. "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 was not availabe for comment.