Tim Gardner, assistant director of policy at the Health Foundation, an independent thinktank, said: "The NHS waiting list often increases at this time of year, but the government is likely to be disappointed not to have made further progress on its commitment to ending hospital backlogs.
"While the NHS was able to maintain a relatively high volume of treatment in July, which included the five-day strike by resident doctors, this is the second consecutive month the waiting list has increased."
The NHS said that it delivered 3% more planned treatments compared with last year with the number of patients joining the waiting list up by 5%.
It also saw its busiest August ever in A&E and for emergency calls to the ambulance service.
NHS Providers, which represents hospitals, ambulance services and other trusts, said staff were "running to stand still" given the increase in demand, and A&E units were also under "relentless pressure".
"If we're going to make a real dent in waiting lists and get more patients seen faster, we need to change how to deliver healthcare," said the body's chief executive Daniel Elkeles.
"The shift to seeing patients closer to home and preventing sickness needs to happen quickly."