Kourtney Kardashian is facing a lawsuit from a model who claims he is her son's biological father.

Michael Girgenti alleges he had unprotected sex with the reality TV star after they met on a photoshoot in August 2008 and has filed paperwork at Los Angeles County Superior Court in a bid to get joint custody of her three-year-old child, Mason.

According to gossip website TMZ.com Michael wants the judge to order DNA tests for Kourtney, Mason and her long-term boyfriend Scott Disick, Mason - who is father to her daughter Penelope, 13 months. A hearing has been set for next month.

The lawsuit documents the alleged relationship between Kourtney and Michael, which he claims heated up in March 2009, when she told him her relationship with Scott was ''on the outs'' and ''she wanted to hang.''

Michael claims they had full sex and as soon as she had given birth to Mason - born on 14 December 2009 - he tried contacting her.

He told In Touch magazine: ''After [Mason] was born, and I saw photos of him, I began to wonder.

''I would definitely want to acknowledge Mason if he's my kid - he deserves to know his real father.''

Kourtney has yet to comment on the allegations.