Heath Ledger was committed to sobriety at the time of his death, according to the sister of his model girlfriend Gemma Ward.

Sophie Ward spent Christmas with the 28-year-old actor, during which time, she says, the Brokeback Mountain star drank nothing stronger than Diet Coke.

She says: "He wasn't drinking any alcohol or taking drugs.

"He smoked cigarettes, but that's about it. He was drinking Diet Coke when we were together and he said he was very committed to not drinking alcohol."

Ward is also convinced that drugs had nothing to do with Ledger's death on Tuesday, despite speculation surrounding his possible substance abuse.

She adds: "He was clean and I don't believe he took his own life deliberately."

Despite this, Ward admits that Heath did seem on edge the last time they spoke: "He couldn't really relax...He said he was going to London but was quite upset because he couldn't see his daughter (Matilda) as much as he'd like to.

"He was travelling so much and I think he was just frustrated with it all."