Mel Gibson may soon be writing one of the biggest checks of his life — to his wife, reports.

With no indication that the estranged couple — who married 28 years ago — had a pre-nup, Robyn Gibson, 53, is legally entitled in their divorce to half of everything the actor-director-producer earned from their marriage in 1980 to the date of their legal separation: A fortune estimated at close to $1 billion US.

“This could easily be one of the biggest divorce settlements in Hollywood history,” says Los Angeles family lawyer Lynn Soodik, who’s not involved with the case. “Any attorney would advise her to take half.”

Among Gibson’s estimated assets: More than $600 million grossed by The Passion of the Christ alone, $100 million-plus in real estate investments worldwide (he bought an island in Fiji for $15 million in 2004), and $75 million for film and TV projects for which Gibson executive produced.

But don’t expect the ex-couple to enter a painful, protracted battle in the courts.

“This divorce will probably reach a settlement in a matter of months,” Soodik says. “It likely won’t be messy since there’s enough money to go around.”