Israel's attorney-general has launched a third criminal inquiry into Ehud Olmert's conduct before he became PM.
Ehud Olmert insists he has done nothing wrong
The PM is already facing investigations into a bank privatisation deal and his purchase of a property in Jerusalem.
In the latest case, he is suspected of granting political favours when he was industry and trade minister. Mr Olmert has denied any wrongdoing.
The cases have raised questions about his future as he prepares for next month's Middle East peace conference.
The justice ministry on Sunday said Attorney-General Menahem Mazuz would investigate allegations of favouritism by Mr Olmert when he was minister between 2003 and 2005.
He is suspected of appointing political allies to positions throughout the industry and trade ministry.
He is also alleged to have helped push through a grant for a business linked to a former associate.
Last week Mr Olmert was questioned by police over allegations that he tried to influence the sale of the state's controlling interest of Bank Leumi in favour of a friend in 2005.
The prime minister has said he persuaded officials to make several changes to the tender to encourage strategic investors to bid for the bank.
Last month, the attorney-general opened a criminal investigation into his purchase of a house which a government watchdog concluded was sold below market value by a developer.