Michael Gove and his advisers are being investigated by the Information Commissioner following allegations they used private email accounts to avoid Freedom of Information (FOI) requests.
The practise of Department for Education (DfE) officials using private email accounts to communicate was revealed by The Huffington Post UK in August.
Leaked emails appeared to show a DfE official using his Hotmail account to put pressure on a primary school in England to convert to academy status.
The official's emails were sent from his Blackberry, and were a clear breach of the rules issued to officials in department, which state:
Never use non-DFE e-mail services (such as your own personal internet e-mail account on Hotmail, Gmail, Yahoo Mail, AOL, MobileMe, etc.) to carry out departmental business.
The investigation was initiated after the Financial Times claimed on Tuesday that it had seen further email traffic between the education secretary and his advisers in which they discuss government business.
Civil servants were subsequently unable to provide the newspaper with the emails when asked for under FOI requests, as they avoided the official email server.
While it is not against the law to conduct official business via a personal email account as long as it is disclosed, it is illegal to do so if the intention is to deliberately avoid FOI requests.
One email, reported to have been sent last December by Gove to his officials, was sent from an email account registered to the education secretary's wife Sarah Vine.