Patients in England cannot rely on information on waiting times for non-emergency operations, such as knee and hip replacements, a watchdog says.
The National Audit Office (NAO) found wrong and inconsistent recording after reviewing 650 cases in seven trusts.
The watchdog said it was unable to discern whether this was deliberate, but overall the practices concealed delays rather than over-recorded waits.
The government said the issue would be investigated.
The NAO said that the lack of reliability, whatever the causes, was harmful to patients because it hampered their ability to make informed choices about where to choose to have their treatment.
It also called into question whether the NHS was actually meeting its waiting-time targets.
There are more than 19 million referrals for elective operations each year.
Patients are meant to be treated within 18 weeks of a referral and the NHS is currently meeting its targets on this - but only just and the report noted there was growing pressure on waiting times.
The waits are monitored and recorded by hospitals themselves.
They have the power to pause the clock if a patient is unavailable for appointments for personal or social reasons.
The patient can also be sent back to their GP - which means the clock starts all over again - if they fail to attend appointments.
But the watchdog said this leeway was being applied wrongly or inconsistently.
Of the 650 orthopaedic cases reviewed, the watchdog found that more than half were "not supported by documented evidence or were incorrectly recorded".
- In 281 cases, waiting times had been correctly recorded and were supported by documented evidence
- In 202 cases, waiting times were not supported by enough evidence to say whether they had been correctly recorded
- In a further 167 cases, there was "evidence of at least one error, leading to under and over-recording of waiting time", with an overall under-recording of three weeks per patient
The NAO also highlighted other cases that were brought to its attention during the review.
These included North West London Hospitals Trust failing to record the waiting times of 2,700 of its inpatients, while Barnet and Chase Farm failed to properly monitor more than 2,000 patients. The problems have now been rectified.
There is also the well-publicised case of Colchester Hospital's misrecording of cancer waiting times.
The report urged NHS England to apply greater scrutiny to waiting-time statistics to establish what was really happening.
NAO head Amyas Morse said: "Performance information should be reliable.
"However, we have found significant errors and inconsistencies in how trusts record waiting times, masking a good deal of variation between trusts in actual waiting times."
Katherine Murphy, of the Patients Association, said the findings were "concerning", accusing the NHS of deliberately manipulating the figures.
"The care and welfare of patients must always come first, and managers' efforts to try and 'fudge' their figures to meet targets will inevitably divert attention from providing high-quality care for the patients."
Shadow Health Secretary Andy Burnham said: "What this report reveals is real, huge questions now over what we are being told by the government about the NHS and waiting times.
"I'll be writing to Jeremy Hunt today to say there must be an urgent review into NHS waiting time statistics so that the public can have confidence in them."
NHS England said action would be taken.
"We firmly believe it is essential to have accurate information provided in a timely way to ensure better care for patients," said NHS England's director of policy and strategy, Bill McCarthy.
A spokeswoman for Health Secretary Jeremy Hunt said: "It's crucial that reporting is always accurate, and we will work with the NHS to make sure hospital staff get the support they need to get this right."