Italian magistrates have opened an investigation into possible murder after the death of a Moroccan model who was a regular guest at former prime minister Silvio Berlusconi's "bunga bunga" parties.

33-year-old Imane Fadil died on 1 March, a month after being admitted to a Milan hospital with severe stomach pains.

At the time she told friends and her lawyer she had been poisoned. Her death was only reported today.

"The doctors have not identified with any certainty any pathology which can explain the death," Milan chief prosecutor Francesco Greco said, adding there were "several anomalies" in Ms Fadil's medical records.

She testified at the 2012 trial of Mr Berlusconi, who was accused of paying for sex with an underage prostitute.

He was initially convicted in the case but ultimately acquitted after a judge ruled he could not have known the prostitute was a minor.

However, magistrates subsequently laid new charges against Mr Berlusconi and other defendants, accusing them of bribing some of the women who attended the parties to keep them from telling the truth at the initial trial. They have denied the accusations.

Ms Fadil was never accused of taking bribes. 

Italian media reported that she was writing a book about her experiences and the magistrates had obtained a copy of the draft following her death.