Italian prosecutors have launched a murder investigation after a key witness in former prime minister Silvio Berlusconi’s upcoming bribery trial died of suspected poisoning.

Imane Fadil, 34, a Moroccan model who attended the media magnate’s notorious “bunga bunga” sex parties, was admitted to a Milan hospital last month with severe stomach pains.

She told friends and her lawyer she believed she had been poisoned.

Fadil died on 1 March, but her death was not announced until Friday.

“The doctors have not identified with any certainty any pathology which can explain the death,” said Milan’s chief prosecutor, Francesco Greco. He said there were “several anomalies” in Fadil’s medical records.

In pictures: Silvio Berlusconi's political career Show all 7 1 /7 In pictures: Silvio Berlusconi's political career In pictures: Silvio Berlusconi's political career 1994 The billionaire media tycoon wins elections with his Forza Italia (Go Italy) party, following a wave of anti-corruption investigations that decimate the old political order. He is forced to step down just months later after his coaltion partner pulls out. Getty In pictures: Silvio Berlusconi's political career 2001 Berlusconi wins elections for a second time after a powerful media campaign in which he promised to slash taxes and unemployment. He goes on to serve the longest stint as Prime Minister in Italy's post-war history Getty In pictures: Silvio Berlusconi's political career 2006 Berlusconi steps down after being narrowly defeated by a centre-left coalition led by Romano Prodi, right, a former president of the European Commission. Getty In pictures: Silvio Berlusconi's political career 2008 Mr Berlusconi wins a third election victory following the collapse of Mr Prodi's government due to internal disagreements over Italian troop deployments in Afghanistan Getty In pictures: Silvio Berlusconi's political career 2009 Veronica Lario, Silvio Berlusconi's wife, files for divorce and accuses her husband of cavorting with under age girls Getty In pictures: Silvio Berlusconi's political career 2011 Silvio Berlusconi steps down following a parliamentary revolt and a growing number of sex scandals and criminal investigations Getty In pictures: Silvio Berlusconi's political career 2013 Silvio Berlusconi is sentenced for having sex with a then under age 17-year-old prostitute nicknamed Ruby the Heart Stealer. He also loses a second and final appeal over a tax fraud conviction Getty

According to Italian news agency Ansa, toxicology tests found the model had been killed by a “mix of radioactive substances”.

Fadil gave evidence in the 2012 trial of Mr Berlusconi, who was accused of paying for sex with an underage prostitute.

The model had been a regular guest at the former prime minister’s parties, where she said young women stripped off while performing provocative pole dances.

Mr Berlusconi was initially convicted of paying for sex with a 17-year-old girl and faced seven years in prison, but he was acquitted in 2015 after a judge ruled he could not have known the prostitute was underage.

Magistrates have subsequently laid new charges against Mr Berlusconi and other defendants, accusing them of bribing women who attended the parties to prevent them telling the truth at the initial trial.

Fadil was never accused of taking bribes and was expected to give evidence in the new case.

In 2012, she said she feared for her safety after telling prosecutors she was offered money in exchange for her silence about what went on at the parties, which Mr Berlusconi claimed were elegant dinner parties.

According to Italian media, Fadil had been writing a book about her experiences at the time of her death.