Prisoner Ombudsman Brian Coulter posted his resignation statement on his website, saying attempts to establish the "necessary independence" of his office have been "largely unsuccessful".

His action appears to have forced the Government into a retreat on the issue.

Mr Coulter was appointed three years ago to look into complaints by inmates inside Northern Ireland's jails, as well as former prisoners.

In his resignation letter, he said he has battled for greater freedom from the Prison Service - the body he investigates - since soon after he was appointed.

His resignation was sparked by a Bill that would have permanently made the Secretary of State Mr Coulter's boss - when he is also in charge of Ulster's prisons.

Mr Coulter argued that his office should be directly accountable to Parliament, just as the Police Ombudsman is.

He told ministers that the legislation "would entrench the Prisoner Ombudsman's dependence on the Secretary of State". He said the office " must be fully independent of those who have any executive responsibility for the Prison Service".

Mr Coulter said his resignation will take effect on June 30, to give the NIO time to find a replacement.

The Bill he was concerned about has now been altered, and it appears the office - renamed the Northern Ireland Commissioner for Prison Complaints - will be directly accountable to Parliament. However, the Bill is still progressing through Parliament and could be subject to further changes.

The Committee on the Administration of Justice (CAJ) has asked the Northern Ireland Affairs Committee to look into the matter. CAJ director Maggie Beirne told the parliamentary Committee that her group understands Mr Coulter's decision to resign "was not taken lightly".

"We respect the Ombudsman's decision, and believe that government is misguided in insisting essentially that the Prisoner Ombudsman report to the very public bodies that he/she is meant to be monitoring by way of an independent complaints system," she said.

The Committee chairman, Sir Patrick Cormack, said he "is very sorry that Brian Coulter has chosen to resign, especially in view of the fact that the provisions in the Bill to which he was objecting have now been withdrawn" .

In a statement, the NIO said they were sorry Mr Coulter had resigned and said he made "a major contribution" to improving standards for prisoners.