More than 10,000 patients were waiting for routine treatment at Gateshead Health in September, figures show.
The King's Fund, a healthcare think tank, said NHS services are already in crisis and warned new Prime Minister Rishi Sunak that the situation is likely to worsen if budgets are cut.
NHS England figures show 12,427 patients were waiting for non-urgent elective operations or treatment at Gateshead Health NHS Foundation Trust at the end of September – up from 12,242 in August, and 9,910 in September 2021.
Of those, 91 had been waiting for longer than a year.
The median waiting time from referral at an NHS Trust to treatment at Gateshead Health was 10 weeks at the end of September – the same as in August.
Nationally, 7.1 million people were waiting to start treatment at the end of September – a new record.
Siva Anandaciva, chief analyst at the King's Fund, said the Government's fiscal statement, due on November 17, will have a "profound impact on the quality and accessibility of health and care services".
"If NHS budgets keep being eroded by inflation, it is hard to see how ambitious government targets to reduce hospital waiting lists can possibly be achieved," Mr Anandaciva added.
"History has shown us that attempts to protect core NHS budgets at the expense of wider spending on social care, illness prevention and capital investment are short-sighted and can lead to greater pressure on services further down the line"
Separate figures show 1.6 million patients in England were waiting for a key diagnostic test in September – a rise on 1.5 million in August.
At Gateshead Health, 5,681 patients were waiting for one of 12 standard tests, such as an MRI scan, non-obstetric ultrasound or gastroscopy at this time.
Of them, 1,074 (19%) had been waiting for at least six weeks.
Dr Tim Cooksley, president of the Society for Acute Medicine, said the figures show "standards are at an unacceptably poor level" that will deteriorate during the winter and that pressure to deliver care is at an unsustainable level.
Other figures from NHS England show that of 72 patients urgently referred by their GP who were treated at Gateshead Health in September, 52 were receiving cancer treatment within two months of their referral.
A month previously – when 85 patients were referred – 48 were treated within 62 days.
In September 2021, 43 patients were treated within this period, out of 71 that were referred.
NHS medical director Professor Sir Stephen Powis said: "There is no doubt October has been a challenging month for staff, who are now facing a tripledemic of Covid, flu and record pressure on emergency services with more people attending A&E or requiring the most urgent ambulance callout than any other October.
He said pressure on emergency services remains high due to a shortage of hospital beds, but that "staff have kept their foot on the accelerator to get the backlog down".
"We have always said the overall waiting list would rise as more patients come forward, and, with pressures on staff set to increase over the winter months, the NHS has a plan – including a new falls service, 24/7 war rooms, and extra beds and call handlers," he said.