FEARING a heartbreaking train wreck, Joel Madden is urging twin brother Benji to put the brakes on his fast-moving romance with Cameron Diaz, The National ENQUIRER has learned.

The “Good Charlotte” singer, 35, is concerned it’s a case of too serious too soon for his smitten sibling and the 42-year-old 

As reported, the love-struck duo – who’ve only been dating since May – are already talking marriage and babies!

But Joel is worried because “he knows his brother is a sucker for a beautiful woman,” said a friend.

“Joel actually likes Cameron a lot and thinks they’re a good match, but he doesn’t want Benji to rush into something so quickly. It’s way too soon to be talking marriage, and Joel told him so. He’s told him to slow down and enjoy the dating phase, or he could end up making a terrible mistake.”

The lovebirds have been inseparable since being introduced by Joel’s wife, Nicole Richie.

It was love at first sight for Madden – who once dated Paris Hilton and Playboy beauty Holly Madison – and Cameron, whose exes include Justin Timberlake, Jude Law and baseballer Alex Rodriguez.

During a steamy European getaway, photographs showed they couldn’t keep their hands off each other.

Sources say Diaz has moved Benji into her Hollywood Hills home as they try for a baby and plan a summer wedding.

But, said a close source, “Joel reminded Benji that while it’s good to follow your heart, it’s not smart to be diving headfirst, especially if they’re talking about children.””It seems Benji is still awestruck that he’s dating Cameron Diaz, but his brother wants him to get a grip on reality and slow things down before moving forward with long-term plans.”