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Lindsay Lohan's father is adamant the actress was not surprised to hear she has a half-sister because she has known about the love child claims for many years.

The Mean Girls star was told of her half-sister's existence during a live interview last week, when a Good Morning America journalist revealed her dad Michael had taken a paternity test during The Trisha Goddard Show.

The daytime show's test confirmed he is the father of his ex-girlfriend Kristi Horn's 17-year-old daughter Ashley, and Lindsay responded to the news by telling the reporter, "I didn't even hear that, so thanks for the news."

However, Michael is adamant Lindsay knew all about the rumours surrounding his possible love child.

He tells TMZ.com, "(My ex-wife) Dina has been telling our children that Ashley is my daughter for years, without even knowing the results. So how could this be news to our children?"

Michael also insists he was separated from Dina when he fathered Ashley, adding, "Dina and I were separated and living separately at the time for eight months."

The former couple was married between 1985 and 2007 and is parents to four children - Lindsay, Michael, Ali and Cody.