The singer, who had once thrilled the world’s opera houses and lived the life of a Hollywood leading lady, became overcome with bitterness and jealousy when her beauty, her voice and her lovers left her.
She was found dead in her Paris flat 27 years ago, aged 53. Although no autopsy was ever carried out, the cause of her death was put down to a heart attack. But far from dying a natural death, Zeffirelli suggested the soprano was possibly poisoned after surrounding herself with a strange circle of new friends.
He told Italian TV news station TG5: "There is the suspicion that they killed her. In her final days she came under the influence of a group of strange Greeks who were connected to her sister and who Maria hated.
"It’s possible they filled her with substances that killed her. After all her body was cremated immediately after the funeral service."
The Italian director’s claim comes just days after a mystery seller sold Callas’s jewels at auction for more than 1.4 million. The collection included diamond and ruby earrings and an emerald necklace, and is thought to have been put on sale by relatives of Vasso Devetzi, a dead Greek friend of Callas.
Zeffirelli, who is due to receive an honorary Knight Commander of the Order of the British Empire for his services to cinema at a ceremony at the British Embassy in Rome this week, said the jewels had been tainted by Callas’s friend’s behaviour following her death.
He said: "Devetzi abruptly cremated Callas’s body without consulting anyone.
"Two friends have told me Callas wrote a will leaving half her estate to her maid and her driver, and the rest to a Milan retirement home. But no will of that kind was found, I believe it was burnt."
Two years ago Zeffirelli made a film about the singer called Callas Forever starring Jeremy Irons and Fanny Ardant which tells of the last tragic days of her life. Described as having one of the 20th century’s greatest voices she lived the glamorous life of an international superstar, only to die alone.
At the height of her career the child of impoverished Greek immigrants to the United States had appeared at the world’s most famous opera houses, from Milan’s La Scala to Covent Garden in London.
Her fame and beauty attracted the attention of international playboys and ensured she became one of the jewel- draped icons of the 1950s and 1960s. Her first marriage to Italian industrialist Giovanni Battista Meneghini collapsed after she began having an affair with Greek rags-to-riches millionaire Aristotle Onassis.
The two met at a ball in Venice but nine years later, as her voice began to fade and her glamour began to wane, he left her for another icon of the 20th century, Jackie Kennedy.
Callas always remained bitter about how Onassis had left her and her anger was famously exposed when letters she wrote to her voice coach Elvira De Hidalgo were auctioned in Rome two years ago.
Callas wrote: "It’s cruel, it isn’t true, both should pay and both will pay, you’ll see.’’
In another she wrote: "The worst thing is that he didn't say anything to me about the marriage. After I’d spent nine years at his side, I think he was obliged to do that, or at least not to let me learn about it in the papers."
After an attempt at a comeback in the early 1970s Callas faded into obscurity and her remaining years were plagued with her trying to battle weight problems. The cause of her death was put down as a heart attack, but no autopsy was ever carried out.
Following her death her 5 million estate was scrappily carved up in court by friends, relatives and hangers-on but an unknown amount is still held by EMI in London from the rights to her records.
The music giant refuses to hand it over despite claims from the Callas family and from Devetzi’s heir, insisting it will only pay once a legitimate claim to the money has been proved.