Some of the surgical gowns ordered from Turkey amid a row over the procurement of personal protective equipment for the NHS do not meet British safety standards, the UK government has said.
The gowns were among a batch of 400,000 sourced by ministers last month and the RAF flew out to collect them when they were not shipped on the expected date.
But it has emerged 2,400 of the 67,000 gowns that have so far arrived in the UK have failed quality checks.
Only 4,500 have been given to the NHS.
The remaining gowns from the shipment are still being tested.
News of the faulty gowns were first reported by the Daily Telegraph on Thursday morning - prompting the prime minister's spokesperson to say the NHS was speaking to the supplier about getting replacements or a refund.
The Department of Health later clarified that a "small number" had failed tests in the UK and it had only paid an "initial deposit".
A spokesman for the Turkish company which supplied the goods said it had not received any complaints.
Mehmet Duzen, from Selegna Tekstil, told the BBC the company had not had any communication from the NHS, the British embassy in Ankara, or British government officials complaining about the quality of its gowns.
"The fabric we supplied was certified. All the goods were certified," he said, adding that they were ready to respond if there was a mistake.
During the past few months as the UK has tackled the coronavirus pandemic, healthcare workers including doctors and nurses have complained of a lack of adequate kit such as gowns, masks and gloves.
PPE is essential for protecting front-line workers exposed to Covid-19, and without it workers are concerned they could catch or spread the virus.
The Department of Health said it was working "night and day to source PPE".
After several delays, the RAF was deployed to Istanbul to fly the PPE it had sourced from Turkey to RAF Brize Norton on 22 April.
The chief executive of the NHS Confederation, representing health and care leaders, said the shipment from Turkey had been "sizeable" but had only offered a few days supply.
Niall Dickson told the BBC Radio 4's Today programme that NHS bosses understood that the quality of equipment was "not always what it says in the box" but warned that making promises that are not kept "undermines confidence" in the government among frontline staff.
He added that supplies were now "generally better" in hospital but that care homes and GP surgeries still face "some difficulties".
One paramedic, who wanted to remain anonymous, described the PPE shortage as like sending soldiers to fight in a war without guns.
"We are desperately short of gowns," he told BBC Radio 5 Live. "We have got enough aprons - which dinner ladies would use - but the actual gowns, we are definitely short of those.
"You're walking through the door, you're hearing these poor souls coughing and spluttering, and you're thinking, 'am I going to be catching that and taking it home to my wife and kids?'," he said. "It's absolutely terrifying."
Last week, the Department of Health asked hospitals not to order their own PPE but to to rely on the government's national procurement scheme.
But Mr Dickson warned that further delivery problems would encourage some NHS organisations to continue to place their own orders.
Asked about the order from Turkey, Northern Ireland Secretary Brandon Lewis said it was "reassuring" that British experts were "ensuring the best quality of equipment".
He told BBC Breakfast that the government was "working through" a list of about 10,000 UK-based firms which have offered to make PPE, and had received 250,000 gowns from Northern Ireland.
A government spokesperson said: "We have received part of an order of around 400,000 gowns from a private supplier in Turkey.
"While a small number of these gowns have failed tests in the UK, more have passed tests making them suitable for use in the NHS. The majority of items ordered from the private supplier are awaiting testing in the UK and Turkish warehouses."
They added the Turkish authorities also provided 68,000 coveralls when it became clear that an order from a private company was delayed and all of these had passed quality checks.
Correction: This story has been updated after the UK government said that only a "small number" of gowns had failed quality tests and not 400,000 as previously reported in the media