Staff shortages and weaknesses in consistency, accuracy and timeliness of inspection reports have all been flagged up as areas of "concern" by the Public Accounts Committee.
Chairwoman Meg Hillier said there was an "alarming lack of attention to detail" when reports were prepared.
The CQC - set up six years ago - said it was proceeding with improvements.
The CQC's inspection programmes for hospitals, primary care and adult social care services in England are all behind schedule, the MPs' report said.
It was concerned by the CQC's ability to respond quickly and effectively to information received from patients, staff and whistleblowers.
On one occasion an NHS trust told the committee it found more than 200 errors in a draft CQC 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," said Ms Hillier.
The cross-party committee also said the regulator had "struggled to recruit inspectors and analysts" and as a result it was "not meeting the trajectory it set itself for completing inspections".
By mid-April the vacancy rate was 34% for inspectors, 36% for senior analysts and 35% for managers. Staff turnover in 2014-15 was nearly 8%, which is higher than the CQC's 5% target.
And the MPs warned that the CQC was not ready to assume new responsibilities for assessing the efficiency of hospitals in April 2016.
"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," Ms Hillier warned.
"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."
The MPs did also acknowledge that the CQC had made "substantial progress" since its previous report into the regulator back in 2012.
David Behan, chief executive of the CQC, said: "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.
"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."
The Care Quality Commission, the health watchdog in England, is "not yet an effective regulator", MPs say.