Sunday Life can reveal the investigation was launched after it emerged a private memo from the Northern Ireland Prison Service chief was circulated around Maghaberry jail last month.

The email - which is believed to have included criticism of Prison Officers Association chairman Finlay Spratt - was initially sent to one of Mr Maysfield's senior colleagues.

But it was later accessed by the hacker and distributed to prison officers at the top-security jail.

Sunday Life understands that any staff member caught in possession of the email faces immediate suspension.

A senior source at the jail said that the email had caused "serious embarrassment" to prison bosses.

Said the source: "They can't believe this has happened and they are determined to find out who sent out the emails.

"This was a very private message between two people and then the next thing it is all over the jail. Even the inmates know what was in it.

"There was some serious criticism of Finlay Spratt after a meeting he had with Mr Maysfield before Christmas and this hasn't gone down to well with POA members.

"The staff are asking why this was allowed to happen because there could have been more sensitive emails in the system about prison security.

"The prison bosses know this was a serious breach of confidentiality and they don't want it happening again."

A spokesman for the Prison Service confirmed an investigation was under way.

Said the spokesman: "The Prison Service has a corporate electronic filing system with varying levels of access for staff.

"It would appear that some documents which had limited accessibility have been seen by a wider circle of individuals.

"The Prison Service is investigating the circumstances surrounding this issue."

Mr Spratt told Sunday Life that he had not seen any emails from Mr Maysfield.

He added: "I did have a meeting with him [Mr Maysfield] before Christmas, but I am not aware of any email at present.

"He is entitled to his opinion, but if he has criticised me then I wouldn't be long commenting on his views.

"I would like to see this email to see what's in it because I will continue in my job to defend prison officers."

sbreen@belfasttelegraph.co.uk