The Oliver! star stunned the world this weekend by revealing he could be the 11-year-old's biological dad because he donated sperm to Jackson in 1996, following a request from the superstar.
Lester is godfather to Paris, as well as the King of Pop's two sons, Prince Michael, 12, and 7-year-old Prince Michael II, aka Blanket.
He admitted that he only came forward with the revelations because he feels Jackson's mother Katherine has cut him out of the kids' lives since their father died.
And now Oxman, a spokesman and attorney for the Jackson clan, has backed Lester's claims -- because he can't imagine why the Brit would lie about such a sensitive subject.
He tells British morning show GMTV, "Mark Lester is a very straight shooter, an honest man, and when he describes the process by which he was asked to make a donation of sperm, I believe every word he says. Michael Jackson always told me he was the biological father, so you have on one hand Mark saying he might be and Michael saying he might be -- so you have the question who the daddy is.
"Mark always told the truth and I believe he is telling us straight right now."
Oxman admits it is perfectly plausible for Lester to have fathered Jackson's daughter -- because he always enjoyed a close friendship with the late "Thriller" hitmaker.
He adds, "There was nobody who was closer to Michael Jackson and his family than Mark Lester and his children. He was the godfather and his children played with Prince, Paris and Blanket. Michael promised him it would always be that kind of relationship, so I understand he is very upset."
Jackson's ex-wife, Debbie Rowe, is believed to have given birth to both Paris and Prince Michael, while Prince Michael II was born to an unnamed surrogate mother.
(c) 2009 WORLD ENTERTAINMENT NEWS NETWORK LTD. All global rights reserved. No unauthorized copying or re-distributing permitted.