The head of the body regulating health and social care has announced her resignation - as a critical government report into its work is published.
Cynthia Bower said it was "time to move on" from her post as England's Care Quality Commission chief executive.
The CQC came under attack last year over the exposure of abuse of residents at the Winterbourne View care home.
The Department of Health report said it had underestimated its task and could have done more to manage risks.
The CQC was heavily criticised over its handling of Winterbourne View, and accused of failing to act when it was warned of concerns first raised in an investigation by the BBC's Panorama programme.
Three people who worked at the home have admitted ill-treatment but eight cases are still to be heard by the courts.
The CQC has also been heavily criticised during the public inquiry into the failings at Mid Staffordshire NHS Trust, and in reports by the Health Select Committee and the National Audit Office.
The CQC began work in February 2009, bringing together three different organisations and registering more than 40,000 providers.
The Department of Health report said it had made "considerable achievements" and that, over recent months, there had been "significant improvements", with increases in the number of inspectors and more focus on "core duties" of registering and inspecting healthcare providers.
But it added: "The review found that the scale of this task had been underestimated by the CQC and the department, and more could have been done to manage risks during the early years of the organisation's operation."
It added: "The role of the CQC has not been as clear as it needs to be to health and care providers, patients and the public."
The review makes 23 recommendations on how the CQC - and the regulatory system as a whole - should improve.
It suggests the CQC should "raise its game" and work more closely with other regulators, and strengthen capability at executive board level.
And it suggests the Department of Health itself should do more to support the CQC and ensure that it is held to account for its role in regulating health and social care.
Announcing her departure, Cynthia Bower said "After almost four years leading CQC, I feel that it is now time to move on.
"The process of setting up an entirely new system of regulation has been intensely challenging - but we have accomplished an enormous amount.
"We have merged three organisations, registered 40,000 provider locations and brought virtually the entire health and social care network under one set of standards, which focus on the needs of people who use services."
Kay Sheldon, a current CQC board member, said: "This is the right decision".
"I believe Cynthia Bower should have left before now given the serious and ongoing problems the organisation has faced.
"Today's capability review report raises significant questions about senior level competence in the organisation as well as the functioning of the board.
"The persistent failure to address, and at times acknowledge, the problems needs to be tackled if we are to achieve an effective and sustainable regulator that functions in the interest of patients and the public'.
Health Secretary Andrew Lansley said: "I would like to thank Cynthia for her work and leadership and wish her the best of luck for the future.
"Over the last year, we have seen CQC make improvements and respond to the need for enhanced scrutiny and enforcement of standards."
Deputy chief executive of the NHS Confederation, David Stout, said: "We are pleased that the government is more interested in changing how the CQC works and its focus rather than the form.
"There has been too much structural change to regulation. The organisation needs some stability to give it a fair chance of success."
Katherine Murphy, chief executive of the Patients Association improvement had been seen in the CQC's work.
But she said it still needed to make changes: "The CQC must do more to tackle poor care but it can only do this if it radically changes its approach.
"It must make sure they visit a representative selection of hospital services during an inspection, not focus on one or two wards."
"Now is the time to ensure that the care regulator is ready and able to provide quality regulation."