ROBIN WILLIAMS went to his grave haunted by his pal John Belushi’s death – believing he should have saved “The Blues Brothers” star.

In a world exclusive interview, Belushi’s drug dealer Cathy Smith confessed to The National ENQUIRER that she injected the 33-year-old “Saturday Night Live” star with the fatal “speedball” – a mix of heroin and cocaine – that killed him after doing drugs with Robin.

The tragedy occurred in a bungalow of Hollywood’s Chateau Marmont hotel in the early hours of March 5, 1982 after a night of partying.

Only hours before Belushi lost his life, Robin and pal Robert De Niro helped themselves to the funnyman’s stash of cocaine in his hotel room and were among the last people to see him alive.

Smith – known as “Silverbag” because she sold drugs out of a silver purse – told The ENQUIRER she and Belushi met up with Robin about 11 p.m. at a Sunset Strip bar, and Belushi was already “stoned out of his gourd.”

Some time after midnight, she returned with John to the hotel where he “had bags of cocaine stashed in his pockets and around the room.

“Soon after that, Robert De Niro and Robin Williams came bopping in, laughing and shouting, ‘Hi, guys – where’s the blow?’ John handed his friends a bag of cocaine. “They attacked John’s bag – sticking straws in it and sucking the white powder into their noses, looking like Hoover vacuum cleaners.”

The drug dealer said the celebs partied until “John started feeling sick and asked me to kick them out, so I did.”

In Bob Woodward’s 1985 book “Wired: The Short Life & Fast Times of John Belushi,” he quotes Williams asking Belushi if he was “okay” before he left the hotel room.

“Yeah,” John said. “Took a couple of ludes.” Before Williams bolted, he told Belushi: “If you ever get up again, call.”

Smith told The ENQUIRER: “John and I stayed up and kept taking drugs…At 3:30 a.m., I shot up John for the last time.

“He got his coup de grace, which is what he wanted. He had a death wish…

“I killed John Belushi.”

Smith left Belushi’s room that morning before he was found dead by his exercise instructor at 12:45 p.m.

Robin was on the “Mork & Mindy” set that afternoon when he was told his pal had died. The stunned actor gasped, “Pardon me?”

Because of Smith’s interview with The ENQUIRER, police arrested her in connection with Belushi’s death and she pleaded “no contest” to involuntary manslaughter charges. She spent 18 months in prison.

Woodward said Williams voluntarily testified before the grand jury investigating Belushi’s death “under an agreement that he would not be asked about his own drug use.”

But until his dying day, the “Mrs. Doubtfire” star could not escape the memory of his pal’s untimely passing.

Williams said: “If I had known something was going on, I would have helped him. … I admired the s— out of him.”

Added a friend: “Robin was haunted by Belushi’s death. He wondered whether John would still be alive had he stayed that night or whether he would have died with him, too.”