People mixing over the Christmas period will "pour petrol on an already burning fire" in terms of coronavirus, an intensive care doctor has warned.
Dr Dave Jones, the clinical director for intensive care at Prince Charles Hospital in Merthyr Tydfil, south Wales, said the Covid-19 situation was now "far worse" than he had previously imagined.
He told Channel 4 News that the hospital was facing an "incredibly grave situation", with critical care patients having to be transferred to different hospitals for treatment.
"The speed and ferocity that the second wave has hit us with is nothing that we could have imagined," Dr Jones told the programme.
"We are literally at the end of our extreme surge criteria for our management of this."
The hospital is at CritCon 3B, meaning intensive care units are set up in different parts of the site and staffing ratios are not in line with national standards.
Those standards are for one intensive care patient to have one ITU nurse but currently the hospital has one nurse per two patients, which is a "stretch for us to deliver" due to shortages, Dr Jones said.
He told Channel 4 News that staff were trying to transfer critically ill patients outside of the hospital to other intensive care units.
Travelling can be dangerous for patients, who are unstable when they come to the intensive care unit, and increases their risk of mortality and dying, he warned.
Dr Jones said the option is kept as a last resort "but we're having to do that and we've got nowhere to go when the network is full".
There are five wards for patients with coronavirus in the hospital but demand for non-Covid patients is increasing too.
When asked about Christmas, Dr Jones said: "We need to stop everything. We need to accept that Christmas is cancelled this year, that we can't carry on.
"The numbers in the community are baked in, we know that. We know these patients are going to come to hospital and a proportion of those are going to come to ITU.
"We have to stop, we have to get as much control of this virus in the community as we can."
It comes as the latest scientific advice in Wales encouraged families in Wales to postpone their Christmas celebrations to control the spread of Covid-19.
Despite the warnings, the four nations of the UK have maintained their position on allowing three households to mix over the festive period between 23 - 27 December.
Dr Jones added: "My message is that we are full. That you might not need intensive care treatment, but one of your family will.
"And we may be getting to the point to say we all full and we can't accommodate your loved ones.
"My message is, keep the social distancing rules, don't travel unless you have to. I appreciate you want to spend time with your families.
"I haven't seen my family since last Christmas. But the sacrifices that everybody's made this year, it would be a tragedy for people to throw everything away for the sake of five days over Christmas.
"The virus isn't going to give us an armistice, we're not going to have five days of no spread. The Christmas period is going to pour petrol on an already burning fire."
Dr Jones called for a lockdown to be immediately introduced in Wales, where the incidence rate is 351.5 cases of Covid-19 per 100,000 people.
His colleague, Lisa Pickering, a sister in intensive care at the hospital, also urged the Welsh Government to implement a lockdown.
"People are just being way too complacent," she told Channel 4 News. "The five days is just going to be absolutely catastrophic for us."