More than 100,000 patients were waiting for routine treatment at University Hospitals Sussex Trust in April, figures show.
The Society for Acute Medicine said the current picture across the NHS in England – where 6.5 million people are waiting to start treatment – is "unacceptable and unsustainable" for patients and staff.
NHS England figures show 105,308 patients were waiting for non-urgent elective operations or treatment at University Hospitals Sussex NHS Foundation Trust at the end of April – up from 103,073 in March, and 93,379 in April 2021.
Of those, 6,736 (6%) had been waiting for longer than a year.
The median waiting time from referral at an NHS Trust to treatment at University Hospitals Sussex Trust was 14 weeks at the end of April – down from 15 weeks in March.
Nationally, 6.5 million people were waiting to start treatment at the end of April.
Dr Tim Cooksley, president of the SAM, said the healthcare workforce and its capacity are currently the key issues facing the NHS, but that the latest figures show there is no easy solution.
He added: “The current experience for patients with long waits for both emergency and elective care is intolerable and this is causing significant morale injury to clinical and operational staff in NHS and social care who wish to provide high quality care for patients.
“The current situation is unacceptable and unsustainable for patients and staff.
"It is essential that the Government urgently commits itself to the long-term solutions.”
Some 12,735 people were waiting more than two years for hospital treatment at the end of April – nearly five times the number waiting in April last year, but down from a record 23,778 in January.
The Royal College of Surgeons of England said this shows "light at the end of the tunnel".
Tim Mitchell, vice president of the organisation, said: “Surgical teams have been working around the clock to reduce the enormous waiting list which built up during the pandemic.
“However, there are still big challenges ahead. As people return to the NHS, demand is only getting stronger.”
The Government has set the ambition to eliminate all waits of more than two years, except when it is the patient’s choice, by July of this year.
Separate figures show 1.5 million patients in England were waiting for a key diagnostic test in April – a fall from 1.6 million in March.
At University Hospitals Sussex Trust, 17,913 patients were waiting for one of 12 standard tests, such as an MRI scan, non-obstetric ultrasound or gastroscopy at this time.
Of them, 4,964 (28%) had been waiting for at least six weeks.
Other figures show cancer patients at University Hospitals Sussex Trust are not being seen quickly enough.
The NHS states 85% of cancer patients urgently referred by a GP should start treatment within 62 days.
But NHS England data shows just 57% of patients urgently referred by the NHS who received cancer treatment at University Hospitals Sussex Trust in April began treatment within two months of their referral.
That was in line with the proportion in March, and 70% in April 2021 last year.
Professor Sir Stephen Powis, national medical director for NHS England, said the NHS still faces pressures, but added: "Hard-working NHS staff are making significant progress in ensuring people waiting the longest time for care are getting treated."
The Department of Health and Social Care said it is tackling the Covid backlog and rising demand for the NHS with record investment.
A spokesman added: “Good progress is being made on cutting waiting times with a two thirds reduction in the number of patients waiting longest for treatment since February."