The Brokeback Mountain star died at the age of 28 in 2008, and his father Kim has now revealed the actor's sister Kate was the last person his son spoke to before he passed away - and she begged him to rethink his lifestyle.

"The last conversation Kate had with him was this discussion about his medications and she warned him, 'You can’t mix drugs that you don’t know anything about'," he tells "He (Heath) said, 'Katie, Katie, I’ll be fine'. Well, that’s a cavalier boy’s answer. It just put his whole system to sleep I guess."

Ledger's death was ruled accidental, while toxicology reports suggested he died from an "acute intoxication by the combined effects of oxycodone, hydrocodone, diazepam, temazepam, alprazolam and doxylamine".

Kim insists the movie star was not a drug addict, but he only has himself to blame for his untimely death.

"That’s what killed us," he adds, "because he was warned by his sister the night before: 'You shouldn't mix what you're taking for pneumonia with your Ambien'. But most of Heath’s problems were self-induced."

Meanwhile, Ledger's dad also claims his son's work schedule led him to drugs: "He was a young guy that travelled all the time for work," he says. "Even as a two-year-old, he hardly ever slept. He was trying to work and travel and do everything in a short space of time.

"Heath mixed a couple of drugs together with sleeping tablets and he’s gone forever. That’s something we just have to deal with."

Kim's comments come on the same day as the publication of a new Rolling Stone magazine interview with Jared Leto, who insists Heath's Oscar-winning portrayal of The Joker in The Dark Knight is "perfect", adding, "It's one of the best performances ever in cinema."