Waiting times for non-urgent treatment at Great Western Hospitals Trust have risen to their highest level since the start of the coronavirus pandemic, new figures reveal.

With colder weather approaching, experts are warning that the NHS must be winter-proof to prevent seasonal flu and further Covid-19 outbreaks from bringing routine surgeries to a halt.

According to NHS rules, patients referred for non-urgent consultant-led elective care should start treatment within 18 weeks.

But NHS data shows 56% of patients on the waiting list for elective operations or other treatment at Great Western Hospitals NHS Foundation Trust at the end of July had been waiting longer.

That was the highest proportion since March, when 27% of patients had waited 18 weeks or more, and meant 12,813 patients had been on the list for longer than the target time.

The rate was also far higher than in July last year, when 18% of patients had been waiting beyond 18 weeks for treatment. NHS trusts are normally expected to make sure 92% of patients are seen within the maximum target time.

Non-urgent elective operations – such as hip and knee replacements – were suspended during the height of lockdown to free up beds for coronavirus patients, leading to delayed care for many patients across England.

Professor Neil Mortensen, president of the Royal College of Surgeons, said patients who have been waiting months for treatment “cannot afford to wait until next spring”.

He added: “We urgently need to build up our hospital reserves if we are to see this winter through. Flu, together with continuing local Covid-19 outbreaks, must not bring surgery to a standstill again, or thousands more will suffer.”

Nationally, 2.2 million people were still waiting for treatment after 18 weeks in July – the highest number for a single month since records began in 2007.

At 47% of those on the waiting list, it was also the worst performance on record.

Some 83,000 patients had been waiting for over a year, the most for any month in more than a decade.