Around half of accident and emergency departments, polled by the Royal College of Emergency Medicine (RCEM), have said they are "full to bursting" this Christmas.
The vice-president of the RCEM, Dr Ian Higginson, told Sky News' Gareth Barlow on Christmas Eve that the situation for the NHS in the UK is "pretty grim" at the moment.
The group, representing emergency doctors, put a call out to senior managers on Friday night. Dr Higginson said half responded and "all but two of them said that the emergency departments were absolutely full to bursting".
"Normally just before Christmas, we'd expect a bit of a lull. So I'm afraid things are looking pretty difficult out there for our patients and for our staff," he added.
The NHS has warned that hospitals are under severe pressure because of winter flu cases and a so-called "quad-demic", combining respiratory infections with norovirus.
Commenting on the challenges facing the NHS, Dr Higginson said: "We simply don't have enough beds in our hospitals for patients who are admitted as emergencies.
"We don't have enough staff for those beds and we don't have any headroom at all. So if something like flu hits as it has done, it makes a bad situation even worse."
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England 'about 10,000 beds short'
Dr Higginson added he believes the answer is "strategic solutions and strategic investment".
He said: "In England alone, we reckon we're about 10,000 beds short in our hospitals to deal with the predictable, urgent and emergency care... the equivalent of approximately two wards in every hospital."
Recently the RCEM also attacked the "nonsensical" guidance on how to treat patients in corridors - describing it as "out of touch" and "normalising the dangerous".
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Dr Higginson said recent pressures mean "we've got patients all the way through our corridors because we can't admit them to hospital when they need to".
He added: "It may be that their ambulance is outside in car parks because those patients can't get into our emergency departments."
And he argued that social care is "in a really difficult place at the moment" - needing investment to prevent older patients from remaining in hospital longer than they need to.
"When they're ready to leave hospital, they get stuck in hospital, and that contributes to that shortage of beds even more," he said.
Since its election victory in July, the Labour government has acknowledged the NHS needs investment, with Health Secretary Wes Streeting and Prime Minister Sir Keir Starmer saying it is "broken".
In October, Chancellor Rachel Reeves announced a £22.6bn increase in day-to-day spending on the NHS in her budget.
The extra cash will be used to improve hospital buildings, enable more procedures and fund new beds and diagnostic tests.
The government has also promised a 10-year plan, to be published in spring 2025, to deliver "a shift from hospital to community, from analogue to digital and from sickness to prevention".