The health regulator which inspects hospitals and care homes in England has "struggled" since its creation two years ago, a report says.
The National Audit Office found the Care Quality Commission had carried out just 47% of planned reviews between October 2010 and April this year.
The CQC took over the work of three previous regulators in 2009 and has had to implement new monitoring systems.
It said it had been a "challenging period" but that it was now "on track".
The commission is responsible for checking if hospitals and care homes meet minimum standards.
It took over from the Healthcare Commission, the Commission for Social Care Inspection and the Mental Health Act Commission.
The NAO said this shift had "created disruption for providers and confusion for the public".
An additional problem was a lack of staff.
As of the end of September, 14% of posts were unfilled - including 100 inspectors' posts, with the CQC affected by government recruitment constraints, which have now been relaxed.
In its report, the NAO added that the process for registering care providers - one of its core jobs - "did not go smoothly".
The CQC did not meet the timetable for two of the three tranches of registrations, it said. And inspectors were diverted from assessing providers in an attempt to meet that timetable.
This and the staff shortage meant that the commission had completed just 47% of its planned assessments between October 2010 and April 2011.
The NAO concluded that the CQC had not, so far, achieved value for money - and said both the commission and the Department of Health were responsible.'Considerable upheaval'
It is not the first time the CQC has faced criticism.
In September, MPs said patients had been put at risk by the fall in the number of inspections of hospitals and care homes.
At the time, Prime Minister David Cameron urged the regulator to act on the MPs' criticisms.
Its work also came under scrutiny after abuse at the Winterbourne View residential home near Bristol came to light.
Publishing this latest report, Amyas Morse, head of the NAO, said: "Against a backdrop of considerable upheaval, the CQC has had an uphill struggle to carry out its work effectively and has experienced serious difficulties.
"It is welcome that it is now taking action to improve its performance.
"There is a gap between what the public and providers expect of the Care Quality Commission and what it can achieve as a regulator. The commission and the Department of Health should make clear what successful regulation of this critical sector would look like."
CQC chief executive Cynthia Bower said: "Not everything has gone smoothly, but we have learned, reviewed what we do and made changes.
"We are a young organisation and we are still evolving - but I firmly believe that we are making real progress."
A Department of Health spokesman said it was currently reviewing the CQC, and the findings of its review would be published in 2012.
But Margaret Hodge, chair of the House of Commons public accounts committee, said the NAO report raised concerns about whether the CQC was "up to scratch".
She added: "The findings are deeply worrying and highlight significant failures that put patient care at risk.
"There has been too much focus on box-ticking and not enough on crossing the threshold and assuring the quality of care."