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The head of the Northern Ireland Civil Service has said he will retire in August after more than three years in the post.

David Sterling said he had informed his colleagues of his decision on Friday morning.

Mr Sterling has worked in the civil service for more than 40 years, joining in 1978.

He said the last three years have been among the "most challenging and difficult" for the civil service.

The ex-DETI chief alluded to difficulties which came about because of the Stormont stalemate.

Northern Ireland has been without an executive since January 2017, when the governing parties - the DUP and Sinn Féin - split in a bitter row.

"We have found ourselves in the unique situation of working without ministerial direction to keep public services running and deliver the best possible outcomes for our people at a time of unprecedented challenge," he said.

"The enormity of the task has put significant pressure on the NICS and I am proud of the way we have responded.

"Throughout my career, I have been impressed and humbled by the work of civil servants across departments and I want to thank them for the great work they do every day to help make people's lives better.

"It has been an honour and a privilege to lead the organisation during these exceptional times," he added.

During his time in the job Mr Sterling has repeatedly expressed frustration with the lack of an executive.

In May 2018 he said there was a "lack of clarity" on what decisions departments can make in the absence of ministers.

Mr Sterling also appeared before the Renewable Heat Incentive (RHI) inquiry, which was set up because of concern over scheme's huge projected overspend.

He apologised for "multiple failings" by Stormont in NI's flawed green energy scheme.

He offered a "profound and unequivocal" apology for the mistakes made and promised that lessons would be learned to prevent it from ever happening again.

He stuck his head above the parapet when commenting about Brexit, warning a no-deal Brexit could have "grave" consequences for NI.

In a letter in March to Stormont's political parties, Mr Sterling came close to suggesting there may have to be some hardening of the Irish border.

However, his letter drew criticism from Conservative MP Daniel Kawczynski, who accused him of engaging in "operation fear".

UK Cabinet Secretary Sir Mark Sedwill thanked Mr Sterling for "his decades of dedicated public service to the citizens and communities in Northern Ireland".

"He is an outstanding colleague with whom we will continue to work closely in his last few months as Head of the Northern Ireland Civil Service," he added.