Accident and emergency patients in England experienced the worst month of delays this winter in the 13 years since the four-hour target was introduced, a leaked document suggests.
Provisional data passed to the BBC says a record number of patients spent longer than the target time waiting to be seen in emergency wards during January.
The figures, compiled by regulator NHS Improvement, also reportedly suggest a record high for people waiting more than 12 hours for a bed after being seen in A&E.
The target, introduced in 2004, states that 95% of patients must be seen and either admitted or discharged in under four hours.
But the document suggests that of 1.4 million visits in January, only 82% were dealt with within the time frame - while more than 60,000 people waited for up to 12 hours for a bed after being told they must be admitted, according to the BBC.
NHS Providers chief executive Chris Hopson told the broadcaster: “These figures have not been verified and should therefore be treated with caution, but they are in line with the feedback we have been getting from trusts.
“NHS staff have responded magnificently to increased winter pressures, but the situation has become unsustainable.
“The rise in long trolley waits is particularly worrying, as there is clear evidence they can lead to worse outcomes for patients.”
Hospitals have not hit the target nationally since summer 2015.
A Department of Health spokesman stressed that the data was yet to be verified and said that official figures for December were only due out on Thursday morning.
He said: “We do not recognise these figures - it is irresponsible to publish unverified data and does a disservice to all NHS staff working tirelessly to provide care around the clock.
“Despite the pressures of winter the vast majority of patients are seen and treated quickly and hospitals have detailed plans in place to manage busy periods - supported by an extra £400 million of funding.”