An autopsy has confirmed that James Gandolfini died of a heart attack, a friend has said.

The Sopranos actor died this week in Rome, where he had been on holiday, at the age of 51.

Friend Michael Kobold has now revealed that a heart attack was the cause and that there was no evidence of foul play or substance abuse.

Asked if Gandolfini had a history of heart problems, Kobold said he was healthy.

"There's nothing out of the ordinary. It was a heart attack. It was a natural cause," he said. "There was no foul play, no substance abuse. None of that."

He said the actor's body has been released to a funeral director and that the family was working to get through the red tape to get the body back to the United States. While the process can take up to 10 days, Kobold said the family was hoping to have the body repatriated by mid-week with a funeral planned in New York by Saturday at the latest.

Gandolfini died on Wednesday night after being discovered in a Rome hotel room by a family member.

He had arrived in Rome on Tuesday and spent his first full day in the Eternal City with his son visiting the Vatican and dining in the hotel, the luxury Boscolo Exedra.

"He had a wonderful day," Kobold said.

Gandolfini was en route to the Taormina Film Festival in Sicily, where he was scheduled to give a special class on Saturday. The festival has now announced that it is organising a tribute to the actor.