David Harker will step down as chief executive of Citizens Advice in May after 13 years in the post.
A spokesman for Citizens Advice, the national body that supports about 400 local bureaux, said Harker, 58, felt both he and the organisation needed a change.
The spokesman added that the timescale would enable a new chief executive to be in place before the next general election, which must take place by early June.
Harker joined Citizens Advice in 1997 from deafblind charity Sense, where he was managing director. He received an OBE for services to the voluntary sector in 2003.
"His commitment to the principles, values and practice of our service is unrivalled," said John Gladwin, chair of Citizens Advice.
Adverts for the job will appear this month and an appointment is expected early in 2010.