Pressure of patient numbers caused a third of the 150 English hospital trusts to warn they needed urgent action to cope last month.
In the worst cases, seven of the 50 trusts that issued alerts announced they were unable to give patients comprehensive care.
Nigel Edwards, chief executive of the Nuffield Trust, which collated the figures for the BBC, warned the situation could deteriorate further in the next two weeks.
He told BBC Radio Four's Today programme: "Anecdotally what you are hearing from chief executives of trusts is that they are experiencing very, very high levels of pressure in their A&E departments.
"The growth seems to be way above what you might expect from what's happened from the population.
"People feel that they can't get to see their GP. The population is getting older and sicker, but I don't think that's the entire reason behind it.
"The other big pressure is the ability to discharge patients. If you can't get patients out of the back of the hospital, home, then obviously, the whole system begins to seize up.
"As we know there are major problems in social care. But also NHS community services. The ability of hospitals to get people out is still really tricky.
"We felt, given the numbers of delayed transfers of care, this number of patients waiting to go elsewhere, the increase in norovirus, the obvious increase in activity that we have seen over the past year, and pressures on the workforce, which are absolutely unremitting, they are reasons to be really quite concerned.
"The real crunch point generally comes in week two or three after the Christmas break.
"I think there are early signs that there is a problem."
The situation has resulted in operations being cancelled and patients left waiting on trolleys, and The Royal College of Emergency Medicine warned patients in affected hospitals could face delays in receiving pain relief and antibiotics, according to the BBC.
Liberal Democrat health spokesman Norman Lamb said: "When pressure on hospitals begins affecting patient safety, the Government has a responsibility to act.
"Health and social care services urgently need a cash injection to help free up beds and prevent this crisis from worsening."
Dr Taj Hassan, president of the Royal College of Emergency Medicine, told the BBC: "Our emergency departments and our hospitals are in an absolute acute state of distress, and this is on a background of chronic under-funding, under-staffing, both in health and social care, and really also failures in the wider urgent emergency care system."