At least 6,000 people are estimated to have suffered "severe harm" as a result of delays of more than an hour in ambulances handing over patients to A&E teams last month, according to a report.
The Association of Ambulance Chief Executives (AACE) said as many as 57,000 experienced "potential harm".
"Longer patient handover delays reached unprecedented levels in December, with the 227,000 hours lost double that of December 2021," the AACE report states.
Official NHS data shows 44% of ambulance patients waited at least 30 minutes to be handed to A&E teams in the week between Christmas and new year - the highest on record.
Some 26% of ambulances waited over an hour in the week ending 1 January.
The AACE report said some 36,000 patients were delayed at least two hours, with 23,000 waiting more than three.
It said the average handover time nearly doubled over the last year, from 29 minutes in December 2021 to 55 minutes in December 2022.
Martin Flaherty, AACE managing director, said: "Our December 2022 data for handover delays at hospital emergency departments shows some of the worst figures we have recorded to date and clearly underlines that not enough is being done to reduce and eradicate these dangerous, unsafe and harmful occurrences.
"At a national level, the average handover time has nearly doubled over the past 12 months, increasing from 29 minutes in December 2021 to 55 minutes in December 2022.
"However, it is particularly worrying that longer delays - those that continue over one, two and three hours - reached unprecedented levels in December.
"The impact on patients is significant and we estimate that around 57,000 patients experienced potential harm as a result of long handover delays in December 2022, with around 6,000 of these experiencing severe harm.
"This does not take into account patients in the community who we were unable to attend due to resources being held at local emergency departments, so the real figure is likely to be much higher.
"We are also acutely aware of how difficult these current working conditions are for ambulance service employees and the considerable impact they are having on their mental health and wellbeing.
"We know that there are good examples of hospitals that are managing their handover delays effectively, and we are doing all we can to help share best practice when it occurs, but now is the time for firm leadership on this vital issue if we are to reduce and eradicate these tremendously damaging delays."
Commenting on the report, Sara Gorton, head of health at the union Unison said: "On the government's watch, the ambulance service has gone from being under pressure during the winter to barely coping all year round. No wonder people are leaving faster than new recruits can be appointed.
"A better deal on pay and a serious plan to tackle staffing shortages can't come soon enough."
The NHS has said it faced a "twindemic" of flu and COVID while also struggling to discharge mentally fit patients from hospital into social care.
A Department of Health and Social Care spokesperson said: "We are taking action to improve access to urgent and emergency care - including investing up to £250m to free up hospital beds, alleviate pressures on A&E and unblock delays in handing patients over from ambulances.
"This is on top of £500m to speed up the safe discharge of patients and creating the equivalent of 7,000 more beds as well as establishing 24/7 data-driven system control centres in every local area to manage demand and capacity.
"Hours lost to ambulance handover delays fell by over 61% in the week ending January 15, the lowest figure so far this winter, and the NHS will shortly set out detailed recovery plans for urgent and emergency care to reduce waiting times for patients."