The number of patients waiting six months or more for surgery has tripled over the past four years in England, the Royal College of Surgeons has said.
In March, nearly 130,000 people had been waiting for operations after being referred to a consultant, compared with 45,000 in March 2013.
But nine out of 10 patients were still treated within 18 weeks, data shows.
NHS England said it had cut the number of patients waiting more than a year for treatment.
A spokeswoman said: "The NHS has cut the number of patients waiting more than a year for treatment by nearly 13,000 over the past five years, and spending on non-urgent surgery is continuing to rise."
However, the surgeons said they were concerned that many more patients would wait longer for surgery in the future.
Earlier this year, NHS England boss Simon Stevens said growing pressures in the health service meant he could no longer guarantee treatment within the 18-week target time for operations such as knee and hip replacements.
The Royal College of Surgeons says this is resulting in more people waiting for six to nine months or more for surgery, in specialities such as ear, nose and throat, brain and spinal, and general surgery.
The biggest increases in waiting times have been in dermatology and gynaecology.
In March, in England, nearly 20,000 people had been waiting for more than nine months for surgery - three times more than in March 2013.
At that time, six-month waits were at their lowest level and 94% of people were treated within 18 weeks.
The average waiting time for planned surgery is now just over six weeks, with 90.3% or 3.3 million people treated in under 18 weeks.
Wales, Scotland and Northern Ireland have also seen increases in the numbers of people waiting more than six months for planned surgery since 2013, although the figures are not directly comparable because of the way they are measured.
NHS England did not respond directly to the six-month figures, but added: "While the Royal College of Surgeons understandably lobbies for more spending on surgeons, in the real world they aren't the only call on constrained NHS funding, which also has to support extra investment in GP services, modern cancer treatments, and expanded mental health services."
Clare Marx, president of the Royal College of Surgeons, said: "We are now struggling to meet the standards and timeliness of care that the public rightly expect.
"It is unacceptable for such a large number of patients to be waiting this long in pain and discomfort for treatment. This is the grim reality of the financial pressures facing the NHS."
She said many of those patients were older and waiting longer for surgery could have an impact on their quality of life and how well they recovered after surgery.
One major reason for the rise in waiting times was a shortage of beds in the NHS, she added.
"When pressures in emergency departments rise, patients awaiting planned surgery can have their operations cancelled or delayed until more space becomes available.
"In this election we urge all political parties to make timely access to surgery an urgent priority."
A Conservative spokesman said the figures failed to recognise the hard work of NHS staff.
"There were nearly 12 million NHS operations done last year, up by almost two million a year since 2010, at the same time as we've dramatically cut the number of people waiting over 52 weeks for treatment," he said.
Jonathan Ashworth, Labour's shadow health secretary, said: "The Tories' neglect and underfunding of the NHS has pushed services to the brink.
"By properly funding the NHS we will restore the 18-week treatment target which the Tories have abandoned and we will bring a million people off the waiting list."