Northern Ireland's Victims' Commissioner is to leave her post for a new role in England.

The announcement of Kathryn Stone's unexpected departure comes just over 18 months after her appointment to the high-profile and politically sensitive position.

She was advocate for the victims' sector during a period of on-going contention over how Northern Ireland deals with the legacy of its troubled past.

Ms Stone, who will step down in June, was at times highly critical of the standard of services offered by the state to those bereaved or injured in the conflict.

But she has insisted frustration at lack of political progress was not a factor in her decision to leave.

"I have not reached this decision lightly," she said.

"And it has, unquestionably, been the most difficult decision of my professional life and possibly the most emotionally taxing.

"I am very sorry to be leaving a place that has become my home and leaving people who have become close friends."

Stormont First Minister Peter Robinson and Deputy First Minister Martin McGuinness have paid tribute to Ms Stone's work and said they were sorry to see her go.

"She has been tireless in her efforts to ensure people, families and groups were afforded a quality service, in acknowledging the legacy of the past and in building a better future," the ministers said in a joint statement.

The leaders said they were committed to implementing Ms Stone's recommendations to improve Northern Ireland's Victims and Survivors Service.

The outgoing commissioner said it had been an honour to work with victims and survivors.

A spokeswoman for the commissioner said she had been offered the opportunity to work in "another critically important area of public life" and said an announcement on her new role would be made shortly.

The ministers said the recruitment process to find a new commissioner would start as soon as possible.

Ms Stone was a former chief executive of learning disability charity Voice UK.

Before that she was previously principal inspector for Milton Keynes Council and head of inspection for the London boroughs of Hammersmith & Fulham and Barking & Dagenham, responsible for the regulation of residential, nursing and day care.

Ms Stone also worked as an independent inspector for eight local authorities across the UK.

She has also been a member of the Home Office Victims Advisory Panel (2006-10) and a member of Derbyshire police's independent advisory group.

Ms Stone was awarded an OBE in 2007 for her services to people with learning disabilities.