DIANA ROSS has become a Supreme pain in the neck as she takes over plans for her 25-year-old son Evan’s wedding to singer ASHLEE SIMPSON!

Sources say the 70-year-old diva is dishing out orders as she takes control of almost every facet of the festivities.

“Diana is already proving to be a monster-in-law,” declared an insider. “She’s driving Ashlee batty with her control­ling ways and wacky demands.”

But sources say Ashlee, 29, has only herself to blame, because she asked for Diana’s help – and now the veteran entertainer has turned into the Wedding Planner from Hell.

“For starters, Diana is insisting the wedding be held at her Greenwich (Conn.) home even though Ashlee has her heart set on Los Angeles or Hawaii,” said the insider. “Diana also wants to stand by Evan’s side during the ceremony – and she’s insisting on singing during the nuptials AND at the reception.

“After Ashlee and Evan say their vows, Diana plans to sing an a cappella version of ‘Endless Love.’ Then, at the reception, she wants to do a 30-minute set of her songs.

“And even though Ashlee wants a vegan wedding cake, Diana says, ‘No way!’”

Making matters worse, sources say that Diana has picked out a showy ivory-colored gown that could outshine the bride’s!

“To top things off, Diana is insisting that Ashlee call her Miss Ross,” revealed the insider. “It’s gotten so bad Evan has had to turn into a wedding referee.”