Some hospitals are failing to accurately monitor how long thousands of their patients are waiting for treatment, a report has found.
Research by the National Audit Office (NAO) discovered trusts in England were often "mis-recording" data, with some saying either patients waited less time than they did for treatment, or longer.
NHS targets say 90% of admitted hospital patients should start their treatment within 18 weeks.
Of those patients who do not need admitting to hospital, 95% should be seen within 18 weeks of referral by their GP.
Today's report said mis-recording of data was identified at The North West London, Barnet and Chase Farm and Colchester hospital trusts.
It said: "The North West London Hospitals NHS Trust identified that it had failed to record properly the waiting times of 2,700 (60%) of its elective (pre-arranged) inpatients, including 12 who had waited more than 52 weeks for treatment.
"Barnet and Chase Farm Hospitals NHS Trust identified that it had failed to monitor more than 2,000 patients on the waiting list, 651 of which had waited between 18 and 51 weeks for treatment."
At Colchester Hospital University NHS Foundation Trust, police are already investigating allegations that cancer waiting times were deliberately falsified.
A Care Quality Commission (CQC) report into Colchester found that, in 22 cases, the treatment dates recorded on the system had been changed.
The NAO report reviewed 650 orthopaedic patient waiting times across seven hospital trusts.
"More than half of these were not supported by documented evidence or were incorrectly recorded," it said.
In 281 cases, waiting times had been correctly recorded and were supported by documented evidence.
But in 202 cases, waiting times were not supported by enough evidence to say whether they had been correctly recorded.
And in a further 167 cases, there was "evidence of at least one error, leading to under and over-recording of waiting time".
"There was an overall under-recording of three weeks (mean) per patient," it concluded.
Today's report said the typical waiting time for patients who had received treatment was "steadily increasing".
It added: "Nationally, the figures for individual trusts show that some do breach the standard.
"In 2012/13, for example, 58 trusts breached the standard, overall, in at least one month for patients admitted to hospital."
The "challenge" of meeting the 18-week target is also rising due to issues such as financial pressures although overall figures show it is being met.
But the NAO said NHS England " does not have sufficient assurance about the performance of trusts".
Without independent validation of trust data, the system currently used by NHS England would "not detect errors or misreporting of the type identified in this report", it stated.
And it said local variations in how the waiting time rules were applied - with some of this variation sanctioned by NHS England - meant the performance of individual trusts was not directly comparable.
The report concluded: " Value for money is being undermined by the problems with the completeness, consistency and accuracy of patient waiting time data; and by differences in the way that patient referrals to hospitals are managed."
The study found that the recent strengthening of the 18-week target to focus on patients who had experienced very long waits "appears to have had a rapid and significant effect on reducing the numbers of people waiting a long time for treatment".
The number of people each month waiting longer than 18 weeks without treatment fell to 138,000 by the end of October 2012 (from 234,000 in October 2011) but by the end of October 2013 had increased to 169,000.
The NAO also spoke to patients who were unaware of their rights and responsibilities, for example, that they may be referred back to their GP and their waiting time started again if they failed to attend an appointment.
Amyas Morse, head of the NAO, said: " The challenge of sustaining the 18-week waiting standards is increasing, against a background of an increasing number of patients being referred to trusts, the financial pressure on the NHS and the need to make efficiency savings.
"If this challenge is to be met, then performance information should be reliable.
"However, we have found significant errors and inconsistencies in how trusts record waiting time, masking a good deal of variation between trusts in actual waiting times.
"The solution is not costly new processes, rather making sure existing processes work properly and are properly scrutinised."
Bill McCarthy, NHS England's director of policy and strategy, said: " It is really good news that the National Audit Office recognise the excellent progress the NHS has made in tackling waiting times for elective care.
"Tremendous progress has been made since the introduction of the 18-week referral to treatment standard.
"More than a million patients are treated by the NHS each month with over 91% of them starting their treatment within the 18-week target and around half of these patients only wait nine weeks or less.
"NHS England firmly believes it is essential to have accurate information provided in a timely way to ensure better care for patients. The NAO rightly points out where there are problems with data and these must be addressed.
"All parts of the NHS have their role to play in ensuring data is collected and recorded accurately including trusts, auditors regulators as well as those who oversee the commissioning of care including Clinical Commissioning Groups and NHS England."
Matt Tee, chief operating officer of the NHS Confederation, told BBC Radio 4's Today programme: "Average waiting times are increasing a bit but they are not increasing by a huge amount and 90% of people are being seen in the target time.
"If the average waiting time is creeping up, then it shows there is pressure on the system and we need to take a look at that and make sure that things are done to deal with that."