CHARLIZE THERON can’t wait to tie the knot with Hollywood hothead Sean Penn – but friends fear she may be headed for big trouble.

Penn, 53, and Theron, 38, have been dating for about four months, and the buzz is they could be Mr. and Mrs. Penn by the end of summer.

“Charlize says this is the single most important relationship she’s ever had and she never wants it to end,” revealed an insider.

But Sean has a history of  being one of TinselTown’s most volatile stars and pals worry that it’s only a matter of time before he explodes.

“Sean has tried to be on his best behavior since he started dating Charlize, but age hasn’t mellowed him,” warned a close source. “He’s a ticking time bomb – you never know when he’s going to go off.”

For now, Penn is playing the part of considerate beau and even doting “dad” to Charlize’s adopted son Jackson. Sources say he plans to adopt the 2-year-old after they wed and jointly adopt a kid from Haiti, where Penn has been heavily involved with humanitarian work.

He’s even been “sucking up” to Charlize’s mom Gerda, who was a victim of domestic abuse at the hands of Charlize’s father Charles, says a friend. But Sean’s violent domestic past – he was once arrested for assaulting his then-wife Madonna in 1988 – is a “very sore point for Gerda,” added the friend.

Sean recently gave Gerda a three-carat diamond neck­lace worth about $50,000 to soften her up, but the friend insists: “Gerda has her sus­picions about Sean and still hasn’t given the relationship her stamp of approval.”

Meanwhile, Penn had a tumultuous 14-year marriage to actress Robin Wright before they split in 2010.

“Friends are urging Charlize to at least have a talk with Robin before she makes any commit­ment,” said the insider. “She knows what a roller-coaster ride it is being married to Sean.”