Fifty million face masks bought by the UK government in April will not be used in the NHS because of safety concerns.
The government says the masks, which use ear-loop fastenings rather than head loops, may not fit tightly enough.
They were bought for NHS England healthcare workers from supplier Ayanda Capital as part of a £252m contract.
Ayanda says the masks met the specifications No 10 had set out. The PM said he was "disappointed" that any protective kit should be unfit for use.
An earlier government statement said its safety standards process is "robust".
Tim Horlick, chief executive of Ayanda Capital, insisted the masks were "not unusable or unsafe" and had met all government standards, adding that No 10 had "not wasted any money" in buying them.
It has emerged that the person who originally approached the government about the deal was a government trade adviser who also advises the board of Ayanda.
But the adviser told the BBC his position played no part in the awarding of the contract.
Asked about the masks, Prime Minister Boris Johnson said he was "very disappointed" that "any consignment" of personal protective equipment (PPE) "should turn out not to be fit for purpose".
Mr Johnson insisted the government had "achieved a colossal race against time" to produce "billions of items" of PPE and source them from abroad, adding that supplies are now being stockpiled in the event of a second wave later in the year.
In the early weeks of the pandemic the NHS experienced severe shortages of PPE.
The government says it had to find new suppliers quickly to meet demand and to compete with rising global competition.
The procurement of PPE is devolved across the four nations of England, Scotland, Wales and Northern Ireland.
The Welsh and Scottish governments said they have not used Ayanda as a supplier.
On 29 April the Department of Health and Social Care signed the £252m contract with Ayanda Capital Limited to supply two types of face masks.
The most expensive part of the order consisted of 50 million FFP2 respirator masks, which are designed to protect healthcare workers from inhaling harmful particles.
According to legal papers seen by the BBC, the government says these masks will now not be used in the NHS because of a safety concern about whether they would fit adequately.
To be effective these types of face mask need to fit tightly to create a seal between the mask and the wearer's face. Anyone who wears them for work is required to undergo a face-fit test.
Alan Murray, chief executive of the British Safety Industry Federation, said products with ear loops fail this test more often than those with head harnesses.
It is not clear what will happen now to the 50 million masks.
Ayanda Capital also supplied 150 million Type IIR masks, which the government says are unaffected.
Most have now been delivered but they have not yet been released for use in the NHS and are awaiting further testing.
Labour leader Sir Keir Starmer said: "For months, we were told that the government was purchasing the right equipment for the front line. Yet again it hasn't happened.
Calling for an inquiry into "what went wrong", Sir Keir said the situation was "just not good enough" for the NHS workers who needed the protective kit.
'Staggering' amounts spent
The information was disclosed in a legal response to the Good Law Project, which is seeking to challenge the government in the courts over three PPE contracts it awarded, including Ayanda Capital's.
The campaign group argues the government's awarding of contracts directly to companies during the pandemic, rather than opening them to competition, may have been unlawful.
Jolyon Maugham, the Good Law Project's director, said it was "important" for a court to examine the contracts to ensure "full transparency of how these staggering amounts of public money have been spent".
One procurement expert described the scramble for PPE at the height of the supply crisis as "like the wild west".
Every leading healthcare system was pressing to secure scarce supplies in the global market as coronavirus infections soared.
With fears that PPE might run out at some hospitals, the UK authorities looked everywhere for readily available masks, gowns and visors.
A Turkish consignment was found wanting in May and now details of contracts with little known companies signed by the Department of Health in England have emerged.
It is understandable that officials facing an unprecedented shortage of vital equipment followed up any offer that looked credible.
But the government has to be held accountable for the spending of taxpayers' money and the Good Law project has highlighted deals that raise questions about due process and transparency.
The government has also disclosed that the original approach to sell the masks came not from Ayanda Capital but from a businessman called Andrew Mills.
His company, Prospermill, had secured the rights to the full production capacity of a large factory in China to produce masks and was able to offer a large quantity almost immediately.
But the legal document seen by the BBC notes that Mr Mills requested the government instead sign the contract for the masks with Ayanda Capital, whose board he advises, because it could arrange overseas payment more quickly.
What do I need to know about the coronavirus?
Mr Mills is also an adviser to the UK Board of Trade.
He told the BBC his position played no part in the award of the contract, which was subject to the same evaluation as all offers made in response to the government's request for help.
A statement from Ayanda Capital Limited said the masks went through a "rigorous technical assurance programme" and could have been rejected through provisions in its contract had they not met government specifications - but those provisions were not activated.
Another government contract the Good Law Project is seeking to challenge in the courts is a £32m deal with Crisp Websites Limited, trading as Pestfix, to supply isolation suits.
In July, No 10 disclosed that none of the suits Pestfix supplied had yet been delivered into the NHS - and instead they were being stored in a Department of Health facility in Daventry awaiting tests.
Now the BBC has discovered the pest control company has separately sold respirator masks to commercial clients using certificates from ECM, a test house in Italy which says it is not a notified body for PPE.
The British Safety Industry Federation said some companies had been presenting so-called 'voluntary certificates of compliance' from ECM but they added these had "no value and no standing" in demonstrating an item of PPE complied with legislation.
Pestfix told the BBC they "unreservedly apologise" for the compliance issue with certain PPE face masks they had supplied to commercial and private clients. It said it is conducting a "thorough investigation" and talking to regulators and affected customers.
The government said in its legal response to the Good law Project that Pestfix provided certificates confirming the suits conformed to the technical standards and they were approved accordingly.