The NHS is at "breaking point" with a decline in the number of hospital beds compounding the situation, according to the British Medical Association.
A reduction in bed numbers has led to delays in admissions and cancelled operations, the BMA claimed.
In a report, based on official statistics, almost three-quarters of NHS trusts were found to have a bed occupancy rate of over 95% on at least one day in the first week of January this year.
The BMA also reported that in November 2016, 14.8% of patients spent more than four hours waiting for a hospital bed, having been seen in an A&E department.
Its findings have been described by Labour as a "wake-up call which Theresa May must not ignore".
The Liberal Democrats, meanwhile, warned that the situation in the NHS was becoming "intolerable".
BMA chairman Mark Porter said: "The UK already has the second lowest number of hospital beds per head in Europe and these figures paint an even bleaker picture of an NHS that is at breaking point.
"High bed occupancy is a symptom of wider pressure and demand on an overstretched and underfunded system.
"It causes delays in admissions, operations being cancelled and patients being unfairly and sometimes repeatedly let down.
"The delays that vulnerable patients are facing, particularly those with mental health issues, have almost become the norm and this is unacceptable."
Mr Porter said that in the short term the NHS needed bed plans that were "workable", but in the long-term politicians needed to "take their heads out of the sand" and provide a sustained solution to the problem.
Department of Health officials have disputed some of the report's findings, however.
Among the figures to be disputed were claims that the number of beds per 1,000 people had dropped from 3.8 in 2000 to 2.4 in 2015.
A DH spokesperson said: "Our hospitals are busier than ever but thanks to the hard work of staff, our performances are still amongst the best in the world.
"We have backed the NHS's own plan for the future with an extra £10 billion by 2020."
But shadow health secretary Jonathan Ashworth said: "Thanks to Tory mishandling of our NHS, patient numbers in hospitals are now routinely above the levels recommended for safety.
"The shameful reality is this overcrowding puts patients at risk and blows apart ministers' claims to be prioritising safety."
Meanwhile, Lib Dem spokesman and former health minister Norman Lamb said: "Chronic bed shortages should be the exception not the rule.
"The situation is getting intolerable, with more cancelled operations, longer delays and those with mental health issues being systematically let down.
"The Government is failing to properly fund the NHS and patients are paying the price."