A total of 5.6 million people were waiting to start treatment at the end of July, according to new figures from NHS England.
This is the highest number since records began in August 2007 and includes those waiting for hip and knee replacements and cataract surgery.
The number having to wait more than 52 weeks to start treatment stood at 293,102 in July 2021, down from 304,803 in the previous month, but more than three times the number waiting a year earlier, in July 2020, which was 83,203.
Health Secretary Sajid Javid has said he was warned the NHS waiting list could reach 13 million without immediate action as he pledged to tackle growing numbers.
Siva Anandaciva, chief analyst at The Kingâ€™s Fund, said: â€œDespite the best efforts of NHS staff, waiting lists for routine NHS care have swollen to levels last seen 15 years ago.
â€œThere are over 5.6 million people waiting for care already, often in pain and dealing with the uncertainty of when they will be treated.
â€œAll NHS services are affected, with primary care, hospital care and mental health services seeing the demand for care rise as the immediate threat of Covid-19 recedes.
â€œEven before Covid-19, waiting lists for treatment had substantially worsened.
â€œThe significant investment the government has now promised is very welcome but will not lead to an increase the number of hospital beds or clinical staff overnight.â€
The data shows the total number of people admitted for routine treatment in hospitals in England in July 2021 was 259,642, up 82% from a year earlier (142,818), although this reflects lower-than-usual figures for July 2020, which were affected by the first wave of the Covid-19 pandemic.
The equivalent figure for July 2019, a non-pandemic year, was 314,280.
NHS England said many more tests and treatments have been delivered this summer compared to last, while hospitals cared for thousands more patients with Covid.
It said there were 3.9 million diagnostic tests and 2.6 million patients started consultant-led treatment in June and July, compared with 2.7 million tests and 1.6 million treatments over the same time last year.