There are now nearly 40,000 patients on the waiting list for routine treatment at Stoke Mandeville and other Buckinghamshire Healthcare Trust hospitals, figures show.
The King's Fund health think tank warned the NHS is "on its knees", as the number of people waiting to start treatment across England reached another record high.
NHS England figures show 37,585 patients were waiting for non-urgent elective operations or treatment at Buckinghamshire Healthcare NHS Trust - within which Aylesbury's Stoke Mandeville Hospital is one of three acute sites alongside Amersham and Wycombe - at the end of September.
That is up from 37,521 at the end of August. It was also 26% more than in September 2020, when there were 29,857 patients on the waiting list.
The median waiting time from referral to treatment was 18 weeks in September, compared to 20 weeks a year previously.
Nationally, 5.8 million people were waiting to start treatment at the end of September – the highest number since records began in August 2007, and the 12th successive record high.
Of them, 12,491 had been waiting longer than two years – more than four times the 2,722 patients waiting this long in April.
Deborah Ward, senior analyst at the King’s Fund, said: “Today’s stats reveal the worst performance since current records began for ambulance calls, A&Es and waits for planned hospital care.
“In a normal year any one of these would ring alarm bells; taken together before winter has even begun, they suggest a health and care system running hot for such a sustained period whilst still dealing with Covid-19, it is now on its knees.”
The Health Foundation said patient care was being impacted by delays to cancer treatment, record waits for ambulances and A&E, and an overstretched workforce.
Tim Gardner, senior fellow at the Health Foundation, added: “With waiting lists at an all-time high, millions of patients and staff are feeling the impact of a health system struggling to cope with demand.
“The Government has said that the NHS is under ‘sustainable pressure’, but it would be extraordinary to look at what’s happening in the NHS right now and claim that it is sustainable.
“That we are facing pressures this significant before we have reached the peak of winter is concerning, particularly as cases of seasonal viruses and rising staff absences are likely to add further pressure.”
Separate figures show 1.4 million patients in England were waiting for a key diagnostic test in September – the highest number since records began in January 2006.
At Buckinghamshire Healthcare Trust, 5,384 patients were waiting for one of 15 standard tests, such as an MRI scan, non-obstetric ultrasound or gastroscopy at this time.
Of them, 912 (17%) had been waiting for at least six weeks.
Professor Stephen Powis, NHS England national medical director said: "Despite high demand, NHS staff are going above and beyond to see more patients and deliver millions more tests, checks, treatments and operations.
“Increasing numbers are coming forward for treatment and this is expected to go up, but it remains really important people do not delay seeking help from the NHS if they feel unwell.”