Stewart, who will take up the role of director of corporate affairs, has considerable experience in public sector comms.
Before his role with GCS, Stewart was briefly head of comms for the Department for Exiting the European Union, where he was asked to set up and run the comms department.
He headed up comms for the long running Iraq Inquiry before joining the Cabinet Office in 2009 and then moved to Defra in 2010, where he led the co-ordinated communications strategy for the 2012 drought.
Before his career in comms, Stewart was a broadcast journalist and covered politics for ITN, Sky News and STV.
Colville, who will take up the job of director of strategy, is currently Defra's principle private secretary, supporting ministers to identify and deliver their priorities, shaping departmental policy, and working across Whitehall.
This has involved the coordination of crisis responses to a range of issues, including flooding and reducing water pollution.
He has worked on infrastructure, EU and energy policy, and in government strategy roles, in both the Department for Transport and the Department for Energy and Climate Change.
Commenting on the appointments, Michael Roberts, Water UK chief executive, said: "We are delighted to welcome Rae and Stuart on board. Their energy, experience and insight will complement the existing strengths in the Water UK team and help us meet the challenges we face in the water sector.
"I look forward to working with them as we continue to enhance the service and support we provide to our members."
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