Kourtney Kardashian was issued with an ultimatum by the male model claiming to be the father of her son Mason.

The reality TV star was recently hit with a lawsuit from Michael Girgenti, who claims he had unprotected sex with her in 2008, but before he took the case to court he reportedly wrote an emotional letter urging her to submit to a DNA test.

According to gossip website RadarOnline.com, in the note dated April 23, he wrote: ''My attentions [sic] are not to hurt the family you've created with Scott. But at the same time, Mason deserves to know the truth.

''I assure you I don't take being a father lightly. I will be a great father to Mason.''

He added: ''Please take this final request for a DNA test seriously because if you don't, I will be forced to take legal action and sue.''

Michael filed paperwork at Los Angeles County Superior Court last week in a bid to get joint custody of her three-year-old child, Mason, and a hearing has been set for next month.

However, the 'Keeping Up with the Kardashians' star insists there is no way Michael is Mason's biological dad, as they never had sexual intercourse.

A source close to Kourtney said previously ''She did not have sex with that model. This whole thing is so ridiculous. She thinks it is stupid and it really isn't true.''

The insider added: ''Kourtney knows who the father of her son is and it isn't this guy.''

Michael wants the judge to order DNA tests for Kourtney, Mason and her long-term boyfriend, Scott Disick, 30.