Michael Jackson's lawyers are battling to save his $900 million secret art collection.

The late 'Thriller' star's "rare intact major collection" has just been discovered, but rather than securing the financial future of his three children - Prince, 13, Paris, 12, and Blanket, eight - lawyers for the star are battling to reverse its $87.7 million sale to an undisclosed international businessman.

Appraiser Eric Finzi told Star magazine the 182-piece collection, which has never publicly been seen, has "an invaluable pedigree for future sales in the international art market".

He added: "Michael's mystique in life combined with this exposure of his wonderful fine art creations following his tragic death will escalate the value of these works and the popularity of his artistic vision worldwide. I do not think we have begun to see the true value of this fine art yet."

The collection is made up of drawings, sketches and sculptures made by the 'Beat It' singer over a number of years while being taught by an Australian artist and friend, Brett Livingston Strong.

Among the artworks are sketches of Martin Luther King, President Abraham Lincoln and President George Washington, said to be worth in all, more than $8.1 million, sketches of Jackson's own feet doing his signature dance move the Moonwalk and recreations of the Statue of David and Dying Slave, the masterpiece of Renaissance sculpture by Michelangelo.

Before Michael died in August 2009 his estate was in financial trouble, yet a letter sent to Brett by Dr. Tohme Tohme - Jackson's last business manager and spokesperson - paved the way for the sale, transferring over the collection of artwork free of charge "to keep, sell, copy, exhibit and to use in whatever way you wish".

Brett and his advisors are thought to have orchestrated the deal to sell off the artwork recently.

Lawyers are now questioning the validity of the letter and trying to reverse the sale.

The letter transferring the works to Brett reads: "Michael wants you to know he is truly grateful for the loyalty you have shown him over the years, and he views this as a small token of appreciation for your continued friendship and artistic partnership."