Important information on the efficacy of new drugs and treatments is going unpublished, posing a risk to health, says a report by MPs.
The House of Commons Science and Technology Committee says despite repeated warnings, not enough is being done to make sure the results of all clinical trials are reported.
Nearly half of clinical trials go unreported, evidence suggests.
It means some clinical decisions are made without all the available data.
In some cases, this might endanger human life, says the group of MPs.
They give the example of heart drug lorcainide, which was tested in 1980.
The results showed that people who were taking it were more likely to die than those who were not, but those findings were not published until 1993 – long after it was made available to patients in the US.
The committee also heard that "publication bias" may have led to UK public money being wasted, for example when the government's decided to spend £424m to stockpile Tamiflu in response to the H1N1 "swine flu" epidemic in 2009.
Dr Simon Kolstoe, a researcher at the University of Portsmouth and chairman of two ethics committees, told the committee: "Eight out of the 10 trials that were used by the company to show the drug was useful in preventing complications such as pneumonia had never actually been peer-reviewed or published."
He said this meant governments were "relying on a marketing spiel claiming successful trials of this drug, rather than being able to consider the actual evidence of the drug efficacy for themselves".
The NHS Health Research Authority (HRA) has been responsible for "promoting research transparency" since 2014, but the committee says not enough has been done to improve reporting rates.
MPs want the HRA to produce a strategy for fixing this problem and say its performance should be measured against progress.
The HRA says it is working with the industry to promote transparency.
Committee chairman Norman Lamb said: "Many of these trials are funded with public money and the taxpayer has a right to expect those who benefit from public funding to follow the rules and publish in full."
He said it was particularly disappointing that trusted bodies such as Public Health England and a range of NHS Foundation Trusts were included in those failing to report results from clinical trials.
A trials tracker website, called the EU Clinical Trials Register, claims that Public Health England has three overdue trials relating to meningitis vaccination.
Dr Mary Ramsay, head of immunisations at Public Health England, said: "We register all our trials on the EU Clinical Trials Register before they begin and are in the process of uploading our results data to it.
"We are committed to transparency and have already published this data publicly in academic journals and online at clinicaltrials.gov."