Milan magistrates yesterday broadened their inquiry into a possible multi-billion euro fraud at Parmalat by launching formal investigations into two of the bankrupt food company's former audit firms, Deloitte and the former Italian branch of Grant Thornton.

The two companies are the first institutions to face official investigation in the inquiry, as Italian prosecutors continue to cast their net wider in attempts to trace those responsible for an estimated €10 billion hole in Parmalat's accounts.

Magistrates in Parma, meanwhile, arrested Mr Franco Gorreri, former chairman of the Banca del Monte di Parma and a Parmalat director until 1992. Mr Gorreri is the 11th individual to have been detained during the investigation. No-one has yet been formally charged.

Neither Deloitte nor Ital Audit, the former Italian arm of Grant Thornton - which was responsible for auditing subsidiaries equivalent to 49 per cent of Parmalat's overall assets - was prepared to comment on their inclusion in the investigation, and both said they had not yet had any official notification from prosecutors.

The firms, which shared the auditing of Parmalat and its subsidiaries, have claimed they were the victims of fraud by Parmalat executives.

Both audit firms were dropped by Mr Enrico Bondi, Parmalat's administrator, soon after he took on the task of rescuing the bankrupt company.

Mr Fausto Tonna, Parmalat's former chief financial officer, and Mr Gianfranco Bocchi, one of its former internal auditors, were both temporarily released from jail yesterday and allowed to return to their offices.

Both had agreed to help sift through files and make sense of remaining documents. - (Financial Times Service)