BBC Trust chairman Sir Michael Lyons is to step down after four years in the post, he announced today.
He has written to Culture Secretary Jeremy Hunt to explain that he will not seek reappointment next May when his term ends.
Sir Michael - the first person to take the role - said the growing workload for the part-time position had made him "increasingly concerned" that it was squeezing out other demands on his time.
In his letter, he said the post "has been far more demanding than the nominal three to four days a week in the job specification".
There has been some uncertainty about the future of the Trust under the coalition Government.
Prior to the General Election, there had been speculation that the Conservatives would move to scrap the body and, as shadow culture spokesman, Mr Hunt had said he had "serious reservations" about the BBC Trust.
The Trust has been credited with helping to make some areas of the BBC's operation more transparent, such as expenses and senior executives' pay.
It has also been involved in a number of high-profile difficulties for the corporation.
Most recently, it launched a public consultation into BBC Director-General Mark Thompson's strategic review.
As a result, the Trust effectively put the brakes on proposals to close the digital station 6 Music.
It was also closely involved with the aftermath of the "Sachsgate" furore, involving Russell Brand and Jonathan Ross.
In his letter, Sir Michael said he had reflected on whether to continue in his position over the summer.
He said the model established for the Trust was "robust, workable and effective".
"I am proud of what we have achieved in safeguarding the BBC's independence against significant challenge, and bringing the interests of audiences in all their diversity to the centre of the BBC's thinking," he said.
And Sir Michael said the Trust had "taken openness and transparency to a new level" and helped to ensure the BBC operated "within clear boundaries".
But announcing his departure, he said: "For all the positives associated with this agenda, I have to acknowledge that the role of chairman has been far more demanding than the nominal three to four days a week in the job specification.
"It is, of course, a compelling aspect of working at the BBC that it can become an all-consuming part of one's life - and this applies equally to the staff across the BBC whose great commitment helps underpin its position as the world's leading public broadcaster.
"But this workload has now reached a point where I am increasingly concerned that it is crowding out other appointments to which I remain committed and other activity that I wish to undertake."
He said he had made his announcement now to leave time to find a successor, and to set a "clear context" ahead of forthcoming discussions on policy issues.
Mr Thompson praised Sir Michael's contribution today. He said: "Sir Michael Lyons has been a tireless advocate for the public interest in everything the BBC does, as well as an effective and dedicated chairman of the BBC.
"But, above all, he has been a vigilant guardian of the independence of the BBC. He deserves our profound thanks for that - as well as a commitment to maintain the principles that he has stood for as chairman."
Culture Secretary Mr Hunt said he was grateful for Sir Michael's efforts.
He said: "I'd like to thank Sir Michael Lyons for the all work he has done during his term as chair of the BBC Trust and wish him the best of luck for the future.
"I am grateful that he has let us know of his intentions in good time so we can begin the process of finding a replacement as soon as possible."
Mr Hunt said he would "look sympathetically" at a request from the Culture, Media and Sport Select Committee to question Sir Michael's proposed replacement before they took the job.
He told committee chairman John Whittingdale that he supported "the idea of more pre-appointment scrutiny for public figures" and particularly those "responsible for policing impartiality in broadcasting".
He said: "I have had no communication on this matter but I would certainly look sympathetically on any request that your committee was to make."
Asked by committee member Labour MP Paul Farrelly whether the next chairman would be a member of the Conservative Party, Mr Hunt said he would not "prejudge" who might apply for the role.
He said: "Let me say this, that the ability to be able to give confidence in exercising judgment with respect to impartiality in the BBC will be a very, very important consideration in the process because impartiality is a very, very important part of what the BBC is about."