The NHS faces "potentially the worst January" ever as it struggles to cope with the backlog of patients taking up beds over Christmas, with a leading doctor warning it is like a "credit card bill from hell".
Dr Mark Holland, president of the Society for Acute Medicine (SAM), said hospitals had already seen "record numbers" of elderly patients over the Christmas period, and that the health service is on the brink of a major crisis.
Dr Holland said hospitals were working under a "false sense of security" as elective procedures fell over the festive period, with those beds becoming available for emergency patients.
Once regular routine operations begin again, he warned, hospitals must make beds available again, and a bout of flu, norovirus or even a cold snap could prove a tipping point for the NHS.
Dr Holland said: "There's this problem over Christmas and New Year where actually, because you get this glut of beds that become available, you're just building trouble up for ourselves in January.
"And then ... when the flu kicks in, if the winter vomiting virus kicks in, which would potentially close a ward, or if there's just a cold snap - when any of these things occur we will be under more pressure, but actually we'll still be spending most of January trying to clear the Christmas and New Year backlog."
This month could be "one of the worst we have faced" as elderly patients cannot be discharged because they are waiting for social care.
Using an analogy of Christmas spending, Dr Holland said: "Imagine, at the end of January, we are going to get our credit card bill for Christmas.
"However, this January it's like we've not paid last month's credit card bill, we've not paid November's credit bill, and over Christmas we've gone out and had a really good spend.
"The credit card bill we are going to get from January is just the credit card bill from hell ... it's going to be the worst ever credit card bill for the NHS."
He added that there were currently "too many unknowns to tell us if we will be able to get through January and avert a major crisis."
He is urging the government to announce its contingency plans in a "possible worst-case scenario".
A Department of Health spokeswoman said: "The NHS is better prepared for winter than ever before, with plans in place to help hospitals cope with additional demand - and the NHS is performing well despite being busier than ever.
"We are committed to delivering a safer, seven-day NHS which is why we have invested #10bn to fund the NHS's own plan to transform services for the future."