The Italian system is "in advance of us in terms of resources and the intensive care beds", he said, and it could be days until some hospitals reach capacity and patients begin spilling out into corridors.
The medic, who works on the front line at a south London hospital and wanted to stay anonymous, said COVID-19 had been "described as the flu" but "really isn't - not the way we are seeing it".
He said people were coming in with "full-blown, really nasty pneumonia".
The only way to get control of the outbreak is through a "fairly strict lockdown", he said, warning that "somebody you know" is likely to be affected.
"We know what's coming, and we know what's coming is extremely big," he said.
He and his colleagues will have to make life and death decisions, allocating resources "only to those deemed most likely to survive".
It is something he suspects will "haunt" them afterwards.
In 30 years as a medical practitioner, the consultant said he had never seen anything like it.
He accused the government of not listening and having a "falsely placed sense of optimism that it will all work out".
Mass coronavirus testing is needed, he said, because self-isolation guidelines are "wreaking havoc".
"We are losing (hospital staff) who are otherwise well," he said.
The consultant has seen Sky News' report from the main hospital in Bergamo, northern Italy, where we found people in terrible distress, gasping for air.
Asked whether he is worried that his hospital could look like that, he said: "Yes, I do - I actually think it could be worse than that."
He added: "I'm aware of the Italian system, and I think in quite a few ways they are in advance of us in terms of resources and the intensive care beds they had available prior to this occurring.
"So I think we may actually end up in a worse state than they presently are unless we take sufficient measures right now."
Asked if he had enough resources, he said: "In terms of what we're anticipating, the honest truth is no.
"The NHS isn't currently set up to ventilate a huge number of people."
Nor are there "sufficient numbers of staff trained in actually managing patients on ventilatory support".
That is "not something you can just teach in an afternoon seminar", he said.
There is not enough protective equipment (PPE) either.
After watching the report from Italy, he said: "I look at the pictures and I can see their staff are in full PPE material - that is not occurring right now in the UK.
"I see that they have got quite a lot of additional ventilators and other machinery which they are able to bring down into their emergency departments.
"We have got sufficient for our intensive care and recovery areas at present."
The worst case scenario, he said, is when intensive care overflow areas become full too, and patients begin spilling out into corridors. "That's what you are seeing in Italy," he said.
At that stage, only the patients thought most likely to survive will be allocated the full resources.
Others, with additional medical problems, will not receive them.
"I think you will then see a really steep rise in mortality," he said.
He added that people "don't stop having heart attacks, people don't stop having strokes, but the resources will not be there for them".
It is "actually quite horrifying to think that we could be completely swamped", he said.