The number of patients in England waiting longer than 18 weeks for routine treatment is the largest since September 2008, official figures show.
The data showed there were over 382,000 patients waiting longer than the target timeframe in April out of a total of 3.78 million on the waiting list.
Problems also continued to be seen in waiting times for A&E and cancer.
The NHS England figures had been due out on Thursday, but were postponed to avoid being published on polling day.
Clare Marx, president of the Royal College of Surgeons, said she expected the situation to get worse given NHS England said earlier this year it was willing to see waiting times increase so it could prioritise A&E and cancer.
“Too many patients are waiting excessively long,” she added.
The figures also showed the four-hour target to be seen in A&E and the 62-day goal for cancer patients to start their treatment were being missed.
NHS Providers chief executive Chris Hopson, said: “These figures show there has been no respite for the NHS. The concerns about the ability of the health service to respond to growing demand – which became so pressing during the winter – are still with us.
“It is clear that trusts are continuing to do all they can. But it is equally clear the situation is unsustainable.
“Despite political uncertainty, we need urgent decisions to ensure the NHS has the capacity to deal with the coming winter, and beyond that, to respond to longer term pressures.”