More than five million people could be stuck on NHS waiting lists for treatment by 2019, documents suggest.
An NHS Improvement presentation leaked to the Health Service Journal (HSJ) estimates a potential rise to 5.5 million from the current list of 3.7 million in a "do nothing" scenario - if the health service fails to takes action.
The current target is for 92% of patients to be treated within 18 weeks of referral by their GP.
But the NHS has not hit this target since February 2016 and performance has been slipping since then.
In March, the head of NHS England said patients will have to wait longer for non-urgent operations.
Simon Stevens said he expects waiting times to rise as a "trade-off" for improvement in other areas, such as hitting the four-hour A&E target and better cancer care.
The latest leaked document said the number of patients waiting more than four months for surgery could more than double to 800,000 by 2019.
The graphs used in the presentation are a "do nothing" scenario in which performance against targets deteriorates significantly over the next two years.
A spokesman for NHS Improvement told HSJ: "Doing nothing is not an option for the NHS.
"These slides were part of a presentation to hospital leaders about steps being taken to improve NHS performance. We are working with providers to improve their overall operational productivity and to help reduce waiting times for patients."
But Ian Eardley, vice president of the Royal College of Surgeons, said: "NHS Improvement's waiting times estimates paint a devastating picture for patients and hammer home just how damaging deprioritising the 18-week target for planned surgery will potentially be.
"Without further help from the next government after the election, this is what the real impact will be on patients of successive underfunding of the NHS.
"Patients need to understand that NHS England's decision to effectively abandon the waiting time target doesn't just mean longer waits in pain such as for hip and knee replacements, it also means unacceptably long waits for more serious heart and brain operations where, in some circumstances, serious disability or even death may result from long waits.
"In Wales, where the waiting time target is 26 weeks, a NHS report previously found a significant number of patients had died plausibly as a result of long waits for cardiothoracic surgery and action was taken to reduce waiting times as a result."
A spokesman for NHS England said: "NHS Improvement describe this alternative as the 'do nothing scenario', but, as their slides rightly point out, in fact the NHS will be doing a lot, hence waits will not move in the way that scenario sketches out."