Police Ombudsman Al Hutchinson has told an assembly committee he will stand down at the start of June next year.
His contract expires in November 2014, but he had previously indicated he would stand down in December 2012.
Mr Hutchinson is addressing the justice committee after a third scathing report about his office.
The Criminal Justice Inspectorate (CJI) report, commissioned by the ombudsman himself, found the independence of his office had been compromised.
Following the report's publication on Monday, Mr Hutchinson said he would quit his job earlier than planned.
Mr Hutchinson said the report had not questioned his office's handling of its day to day business of dealing with complaints against the police, only its handling of historic cases.
He said he was not biased towards the police, but was biased towards good policing. He added that his integrity was intact.
"Reports were not altered to limit criticisms of the police," he said.
Mr Hutchinson said others had sought to attack him and the work of his office.
"After a decade of commitment to improvement of policing in Northern Ireland, I will not let those who want to undermine progress for their own narrow agendas to succeed in destroying this office or indeed in shaping its future to their own ends," he said.
Replying to a question from Sinn Fein's Raymond McCartney about the CJI report's criticism of his office, the ombudsman said he had neither been "asleep at the wheel" nor had anyone else been driving.
Sinn Fein's Jennifer McCann said she was sad Mr Hutchinson was not standing down straight away.
Mr Hutchinson called in the CJI inspectors after the chief executive of his office resigned and criticised how business was done.
Speaking to the committee before Mr Hutchinson's appearance, the Criminal Justice Inspectorate's Dr Michael Maguire told MLAs that investigations into historic cases were compromised by emerging findings being revealed while work was not finished.