Siddall will work alongside the new chief executive, to be named shortly, until the end of the year and will then retire.
Siddall has been Thames’ chief financial officer, an executive director on the Board, and a member of the executive team since September 2011. He has had a 40-year career in corporate finance and has held five successive chief financial officer appointments since 1990.
A search for his successor as chief financial officer will start once the new chief executive is in post.
Thames Water chairman Peter Mason said: “Stuart Siddall has been a strong and effective , playing a major role in increasing the resources directed to front-line services benefitting customers. He has also taken a strong lead in increasing transparency and explaining the company’s finances.
“Stuart has always made it clear that he would want to stand down from full-time roles about now and we wish him well in his retirement.”
Thames also announced that strategy and regulation director Nick Fincham will join the board as an executive director.
Fincham has been the company’s strategy and regulation director, and a member of the executive team, since April 2011. He oversaw the preparation of the company’s business plans ensuring that these were centred on the needs of the customer.
Mason said: “Nick Fincham has been a highly effective member of the executive team, including leading the way in the last regulatory price review, and we are delighted to welcome him to the board.”
Earlier this week it was announced that Graham Southall, managing director of Thames Water Commercial Services, and Rupert Kruger, Thames’ head of business retail, will leave the water company this year.
Last November, Thames announced that chief executive Martin Baggs will stand down this year.
The recruitment process for his replacement is ongoing, and the aim is to have a successor in post by the end of this year. Baggs will stay on until the new chief executive takes over.