Drug companies have "routinely and legally" withheld the results of medical trials from doctors, researchers and patients for decades, British MPs have said.
In a damning report, the Public Accounts Committee said it was of "extreme concern" that about half of all trial results for medicines available on the global market were not subject to public and independent scrutiny.
Warning that doctors and patients are being "undermined" in their ability to make informed decisions on treatment, the committee called on the Government to act to ensure that the results of all clinical trials of every medicine currently being prescribed are made available.
The report will significantly increase pressure on the world's pharmaceutical giants to commit to full transparency over clinical trials.
Critics have accused "Big Pharma" of systematically putting profit before patients, withholding data that might undermine confidence in their blockbuster drugs.
MPs said "trials which gave a favourable verdict" were "about twice as likely to be published as trials giving unfavourable results".
The Association of the British Pharmaceutical Industry (ABPI), which represents leading British firms such as GlaxoSmithKline and AstraZeneca, said it was "misleading" to say data were being routinely withheld and that results from nine out of every 10 industry-sponsored clinical trials for "recently approved medicines" had been published.
Yet committee member Richard Bacon, MP, said results of past trials on medicines still in use today could be of equal concern.
Tamiflu, which for years has been stockpiled by Governments for emergency use in the event of a flu pandemic, caused particular concern to the committee, which said it was "disturbed by claims that regulators do not have access to all the available information" about the drug.
The Department of Health spent 424 million ($850 million) on Tamiflu between 2006 and 2013 - despite "a lack of consensus about how well [it] works", said Bacon.
The MP said the widespread lack of transparency was of "extreme concern" to the committee, adding that recent proposals by regulators and the pharmaceutical industry to open up access to clinical trial results were insufficient.
"The Department [of Health] and the Medicines and Healthcare products Regulatory Agency (MHRA) must make sure, both prospectively and retrospectively, that clinical trials are registered and the full methods and results of all trials are available for independent wider scrutiny by doctors and researchers."
The committee also criticised MHRA and the National Institute for Health and Care Excellence (Nice), the health standards body, for not routinely sharing information from manufacturers when licensing medicines.
Dr Ben Goldacre, a leading campaigner for transparency on clinical trials, who gave evidence to the committee, said the public had been given "false reassurance" by the pharmaceutical industry and the medical establishment for at least 20 years.
"Patients suffer and die unnecessarily because of this," he said. "It's fairly unusual that drugs that do more harm than good come to market, but it's very common that we are persuaded to use the less effective of the available treatments because of withheld trial results.
"When we use the second- or third-best treatment in its class, we are inflicting unnecessary harm on our patients. That's real avoidable suffering and death. That's why it's so tragic that the medical and academic community has failed to address this problem."
The influential British Medical Journal has also been a vocal critic of the pharmaceutical industry's stance.
Its editor, Dr Fiona Godlee, said: "The fact that the majority of clinical trials are funded by the people that make and sell the drugs is something that is hard to understand and yet it's something that is accepted as 'the way things are done'.
"At the moment it's commercial implications that hold sway."
A Department of Health spokesperson said it was working with health organisations to make clinical trials more transparent.
"All clinical trials conducted in the UK must now be registered on a public database.
"Patients have rightly been concerned about confidentiality in trials so we need to balance the need to make trials more open with making sure patient data [are] kept safely."
Tamiflu: a drug shrouded in secrecy
Britain's Public Accounts Committee said it was "disturbed" by claims that regulators did not have access to all the available trial data on the anti-viral drug Tamiflu.
There was "limited evidence and widespread disagreement among regulators and other bodies internationally on whether [it] confers any benefits on complications and mortality", MPs added.
Roche has now committed to publishing full clinical trial results on Tamiflu, and the PAC recommended the Department of Health review whether to spend a further 49 million ($98.2 million) on stockpiles.
Britain's Department of Health has already spent 424 million on Tamiflu between 2006 and 2013.