Wray has been chief executive at the firm since 2005, and will have spent nine years in charge by the time he stands down in spring 2014.
He told the board of his plans several days ago, and the FTSE 100 firm is yet to identify any internal candidates or appoint headhunters to help with the search for his replacement.
The firm said the lengthy notice period allows it to prepare for regulator Ofwat’s next five-year plan before Wray steps down.
“Tony has also been an influential voice within the industry and has played a significant part in setting its future direction,” said chairman Andrew Duff.
Wray is also a non-executive director at housing group Grainger.