More than 100,000 patients were waiting for routine treatment at University Hospitals Sussex Trust in March, figures show.
The Society for Acute Medicine said the latest data shows pressure on the NHS nationally is "unsustainable" and needs urgent action from the Government.
NHS England figures show 103,073 patients were waiting for non-urgent elective operations or treatment at University Hospitals Sussex NHS Foundation Trust at the end of March – up slightly from 102,771 in February, and 52,510 in March 2021.
Of those, 6,367 (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 15 weeks at the end of March – down from 16 weeks in February.
Nationally, 6.4 million people were waiting to start treatment at the end of March – the highest number since records began in 2007.
But the number of people waiting more than two years has dropped for the second month in a row.
Separate NHS England figures show that a record 24,138 people had to wait more than 12 hours in A&E departments in England in April, from a decision to admit to actually being admitted.
Siva Anandaciva, chief analyst at The King’s Fund think tank, said that unless the Government “grasp the nettle” on health and social care staffing shortages, patients will be left waiting in “discomfort, pain and deteriorating health”.
He added: “The top priority for the NHS is to tackle the longest waits, so it is some comfort to see the number of people waiting over two years for planned care starting to come down.
“This week’s Queen’s Speech identified reducing the backlog of care as one of the Government’s top three priorities.
“But the reality check is that until ministers grasp the nettle on health and care staffing shortages, it will be patients who continue to pay the price by waiting longer in discomfort, pain and deteriorating health.”
Separate figures show 1.6 million patients in England were waiting for a key diagnostic test in March – a rise on 1.5 million in February.
At University Hospitals Sussex Trust, 18,936 patients were waiting for one of 12 standard tests, such as an MRI scan, non-obstetric ultrasound or gastroscopy at this time.
Of them, 4,897 (26%) had been waiting for at least six weeks.
Dr Tim Cooksley, president of the Society for Acute Medicine, said: “The latest set of performance data shows that the new “normal” is at an unacceptably poor level for both patients and staff.
“Pressures are at unsustainable levels and, at months where NHS teams hope for a quieter period, worse performance and standards are dominating the horizon."
He added: “This is an emergency which needs recognition, action and support on an urgent basis; it cannot afford to join the waiting list being endured by so many patients.”
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 March began treatment within two months of their referral.
That was up from 49%in February, but down from 69% in March 2021 last year.
The Department of Health and Social Care said it has provided a record £36 billion over the next three years for the NHS and social care, and launched a plan to tackle the Covid backlog.
A spokesman added: “We recognise the unprecedented pressure NHS staff are under from the pandemic – especially frontline ambulance workers."