A delivery of 84 tonnes of personal protective equipment (PPE) for frontline workers has been delayed.
The shipment, which includes 400,000 surgical gowns, was due to arrive in the UK from Turkey on Sunday afternoon.
The Royal Air Force (RAF) is on standby to bring the equipment on Monday, the Government announced.
The reason for the delay is not yet known.
A Government spokeswoman said: “We are continuing to work to ensure the shipment is delivered as soon as possible.”
Speaking at the daily Downing Street press conference on Sunday, Education Secretary Gavin Williamson said the RAF was on standby to bring the equipment from Turkey.
He said: “We hope to see that coming into the country tomorrow (Monday).”
Communities secretary Robert Jenrick set out details of the consignment at the press conference on Saturday, describing it as a “very significant additional shipment”.
He said: “Supply in some areas, particularly gowns and certain types of masks and aprons, is in short supply at the moment and that must be an extremely anxious time for people working on the front line.
“But they should be assured that we are doing everything we can to correct this issue and to get them the equipment that they need.”
Health bosses said the shipment would provide only enough PPE to last “a few days”.
Niall Dickson, chief executive of the NHS Confederation, said it “may be enough to avert an absolute crisis over this weekend” but “it doesn’t solve the longer term problem”.
Responding to the delay, Mr Dickson said: “This is very serious and makes a difficult situation worse.
“It would have been better had the Government not made the announcement in the first place – we know shipments in this supply chain are unreliable and even when they do arrive they are not always what is expected.
“In future, the NHS will expect announcements when supplies have arrived not promises about what may or may not be delivered.”
He said it is likely some hospitals will now run out of gowns but added: “Everything that can be done will be done to protect patients and staff.
“We understand there are other potential sources of supply but – to be clear – nothing is guaranteed.
“There was already a loss of confidence and real anxiety across the NHS and care system around PPE supply – this will have been further eroded by what has happened this weekend.
“It will not be restored until we have a stable supply chain and certainty that staff will be given what they need when they need it. At the moment we are a long way from that point.”
They are being asked to sacrifice themselves due to the failings of othersHCSA president Claudia Paoloni on NHS workers
It comes after the Hospital Consultants and Specialists Association (HCSA) said officials responsible for the UK’s “disgraceful” lack of adequate personal protective equipment need to be held “fully accountable for their abject failure” to protect front-line health staff.
HCSA president Claudia Paoloni said: “Our NHS workers are going above and beyond on a daily basis to heal.
“They should expect at the very least adequate protection to keep them fit and well to engage in this fight.
“Yet instead they are being asked to sacrifice themselves due to the failings of others.”
The Government has been under fire for weeks over the distribution of PPE, with some front-line staff warning they have to work in situations where they feel unsafe.
At least 50 NHS workers have now died after contracting coronavirus.
Dr Chaand Nagpaul, chairman of the British Medical Association (BMA) council, said he had written to the Government two weeks ago calling for a massive ramping up of the manufacturing of protective equipment.
He said large numbers of contacts were willing to produce equipment and names of around 70 of them were passed on to the Government.
But Dr Nagpaul said the contacts “hit a brick wall” after they were not followed up.
He told Sky News: “We made it clear weeks ago that we need to do something about the likelihood of a lack of protective equipment.”
When Mr Williamson was asked why the Government has not responded to manufacturers who are willing to produce PPE, he said they will be contacted in the next 24 hours.
He said a billion extra PPE have been brought into the country while the Government was doing “immense work” in trying to find British suppliers.
He encouraged those suppliers who have been in contact with the Government and have “slipped through the net” to get in touch again.
He said the Government will ensure they are contacted “in the next 24 hours”.
He added: “We recognise this is a national endeavour and we are so incredibly grateful for so many people who are willing to step forward to make a real difference, and we certainly don’t want to miss out on those opportunities.”
New guidance from Public Health England says long-sleeved disposable fluid-repellent gowns should be worn when treating Covid-19 patients.
If the gowns are unavailable, clinical staff have been advised by the Department of Health to wear “disposable, non-fluid repellent gowns or coveralls” or “washable surgical gowns”, with aprons, and to wash their forearms afterwards.
It also says staff should consider reusing PPE when necessary if supplies are low.
A total of 16,060 patients have now died in hospital after testing positive for coronavirus in the UK.