The Royal College for Emergency Medicine said the “dreadful” delays nationally were causing serious harm to patients and driving staff to leave the NHS.
NHS England data shows 771 people arrived at East Sussex Healthcare NHS Trust A&E by ambulance in the week to December 5.
Of them, 70 (9%) waited more than 30 minutes before being handed over to A&E staff, with nine (1%) waiting more than an hour.
The NHS has a target of 15 minutes for ambulance handovers, but only delays longer than half an hour are recorded.
The worst day for delays at East Sussex Healthcare NHS Trust was December 5, when 17% of patients faced long waits – the highest proportion recorded across the week.
Dr Ian Higginson, vice president at the RCEM, said the coronavirus pandemic made the “hidden” issue of hospital crowding visible across the country.
He said: “We used to say that emergency departments were thought of as having elastic walls – we would desperately try to get patients in to release ambulances back onto the road.
“That meant we would bring patients into emergency departments and would end up putting them in corridors, doubling up patients in cubicles, doing all sorts of things that were horrible for patients and led to crowding.”
Dr Higginson said Covid infection control measures meant patients could no longer be crammed into A&E departments during busy periods.
He added: “That meant we had to stop offloading ambulances – a problem hidden inside emergency departments became very visible.”
Lengthy ambulance handover times lead to delays in assessment and treatment and fewer ambulances being available to respond to emergencies, as well as having a knock-on effect on staff, Dr Higginson said.
He added: “There has been a chronic long-term failure in leadership and planning around the emergency care system and the NHS for many years.
“Repeated warnings about this problem have been ignored.”
Charity the Health Foundation said the NHS had little choice but to “weather the winter storm”.
Tim Gardner, senior policy fellow, said: “As well as focusing on hospitals, the recovery plan will need to boost services in primary care and care in the community to keep more people from becoming ill in the first place.”
NHS England said last month, staff answered the highest number of 999 calls for any November on record – an average of around one every three seconds – and it was also the busiest November on record for A&E, with more than two million patients seen.
Professor Stephen Powis, NHS national medical director, said staff were continuing to address backlogs in the face of “sustained pressure” on urgent and emergency care.
He added the NHS was heading into a “challenging winter” and the impact of the new Omicron variant of coronavirus was still unknown, but encouraged people to come forward for their booster jabs.