Patients are being denied proper protection over health and care standards because of a regulator's failure to recruit sufficient staff and its error-strewn reports, a Commons spending watchdog said.
MPs on the public accounts committee issued a stark warning that the Care Quality Commission was still not doing an "effective" job six years after it was set up and demanded rapid improvements.
In a scathing report it said the Commission was "behind on its inspection programme and is not, therefore, fulfilling its duty to be sighted on risks to the quality and safety of health and adult social care services".
More than a third of inspector, senior analyst and managers roles remained unfilled in April - with a full complement not expected until June 2016, leaving its hospital inspection regime at least six months behind schedule.
Funding cuts could further worsen the situation, the report suggested.
The committee raised concerns about the CQC's "ability to respond quickly and effectively to information received from service users and staff" - notably from whistleblowers.
And it criticised the failure to be ready to take on new responsibilities to examine how well hospitals were using resources - which were meant to come into force in April 2016 but would not be fully operational until a year later.
Committee chair Meg Hillier highlighted "an alarming lack of attention to detail" in CQC reports.
The Labour MP said: "Six years after being set up, the Care Quality Commission is still not fully effective.
"There's too often a long gap between inspections and reports being published - and sometimes an alarming lack of attention to detail when reports are being prepared.
"One NHS Foundation Trust told us staff had identified more than 200 errors in a draft Commission report, including data inaccuracies.
"The fact these errors were picked up offers some reassurance but this is clearly unacceptable from a public body in which taxpayers are placing their trust.
"Recruitment at the Commission is going too slowly, meaning too many members of the public don't have up-to-date independent information about the quality of services provided.
"It is vital the public is clear on what the Commission has actually inspected, and when.
"If the Commission is to properly fulfil its duty to taxpayers we must see improvements in the way it collects, acts upon and publishes information.
"At the same time it should set out a coherent plan for managing its new responsibilities.
"When the Commission falls short, there must be robust measures in place to enable Parliament and the public to hold it to account."
CQC chief executive David Behan welcomed the committee's acknowledgement of "substantial progress" since the last highly-critical report by the PAC, in 2012.
But he accepted there remained many areas where performance had to be improved.
"We have always maintained that there is more we have to do, in particular with regards to improving the timeliness of our reports and inspecting all health and adult social care services," he said.
"These are not new issues and we have been working hard to improve our performance.
"We have reported on our progress in public every month and we will continue to do so.
"What is essential is that we do not take any shortcuts, which could compromise the quality of the important work that we do.
"We have surveyed the public and they tell us they are increasingly confident in our new and more rigorous approach to inspection that provides them with better information about the quality and safety of care.
"We are not complacent and are working confidently to continue to improve what we do and how we do it.
"We look forward to reporting to the PAC in the summer on the progress we have made."
The MPs complained that one in three "safeguarding alerts" was not acted on within the Commission's two-day target, reports were issued too long after inspections "and the Commission's draft reports contain too many basic factual errors".
"At a time when the Commission is asking providers to pay substantially more towards the cost of their inspection, it is more important than ever that the Commission can demonstrate the quality of its work," it concluded.
"However, providers told us they find too many errors in draft reports, reports take too long to produce and there is too much variation in the quality of initial judgments."
Delays to the full implementation of the hospital resource watchdog role "risks giving the public the impression the Commission is providing full assurance over the use of resources by all hospitals when it will not be doing so until January 2017", the report said.
Clear performance targets must be set, the MPs concluded.