Lindsay Lohan's father has been branded a liar for denying he is the father of a secret lovechild - by the young girl's mother.

Michael Lohan has admitted to romancing the teenage girl's mother, Kristi Kaufmann, during a period in 1995 when he and his now ex-wife Dina had separated - but has instructed lawyers to find out the truth as to the child's paternity.

Kaufmann claims to have had a year-long affair with the Mean Girls actress' father - and the result was her 13-year-old daughter Ashley.

She had a paternity test in 2005 as she suspected her ex-husband was not the father of the teenager - and has allegedly been proven right.

She says, "The only other person I had slept with was Michael. My daughter has a right to know who her father is - and it's Michael."

Revealing further details of the relationship, Kaufmann adds, "He met me in Houston and we met again in New York."

"I just want to make sure the truth comes out. Michael is already out there spreading lies."

Lohan insists he had no idea that his one-time mistress had given birth to a child after their romance, and Christie hit him with the bombshell news while he was in prison, serving time for a Dui and other charges in 2005.