Health software group iSoft is being probed by the Accountancy Investigation and Disciplinary Board (AIDB), which regulates accountants.
The investigation will look at the role of iSoft's former auditors RSM Robson Rhodes, as well as former directors, including Sir Digby Jones, the former director-general of the CBI, who has since resigned from iSoft.
In August, iSoft announced it was being investigated by the Financial Services Authority for possible accounting irregularities.
ISoft, which supplies software to the national programme to computerise the NHS, is to quit its headquarters building in Manchester and move staff to its Banbury offices.
The AIDB can speak to former directors, but can enforce fines or restrictions only on accountants.
News of the investigation comes after iSoft, which is a key supplier of software to the NHS's £6.2bn IT modernisation programme, announced that it was putting itself up for sale.
Shares in the group have slumped since the beginning of the year, after it announced it was changing its accounting policy and rewriting accounts from previous years, reducing its profits. Its former commercial director was suspended.
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