The Parmalat scandal rocked Italy

They are US banks Citigroup and Morgan Stanley, Germany's Deutsche Bank, Swiss bank UBS, and Nextra, asset management arm of Italy's Banca Intesa.

Parmalat collapsed in late 2003 with debts of 14bn euros ($19bn; �10bn).

Prosecutors in Milan have been probing the role played by banks in aiding the firm to raise cash in bond issues.

The investigation is also reported to have come up with the names of 13 bank executives, including an employee of Credit Suisse First Boston (CSFB), who are suspected of market-rigging.

The announcements bring to an end the second phase of investigations by Italian prosecutors.

The findings have raised the prospect of the Italian authorities calling for prosecutions of the financial institutions.

'No factual basis'

Citigroup said the claim "if pursued against Citigroup, cannot be proven", while UBS called its actions legal and promised to defend itself in any proceedings.

In a separate development Citigroup said on Thursday it was suing Enrico Bondi, the recently-appointed chief executive of Parmalat, on allegations of fraud, negligent misrepresentation, and taking money owed to the world's largest bank.

Its claims are in response to a lawsuit brought last summer by Mr Bondi, Parmalat's then-administrator, in a New Jersey state court. Mr Bondi sued to recover $10bn from Citigroup.

Meanwhile, in response to the new claims in the Italian investigation, Morgan Stanley said there was "no factual or legal basis for seeking any indictment", and CSFB would not comment.

Deutsche Bank said it had done nothing wrong.

Several former top Parmalat executives, auditors and bankers plus three financial institutions were last year investigated for their role in the scandal.

Earlier this month Parmalat said it hoped to be back on the Italian stock exchange in July.

As part of the re-listing on the Italian stock exchange, creditors' debts are expected to be converted into shares through two new share issues amounting to more than 2bn euros.

The company's creditors will be asked to vote on the plan later this year.