The government says it may amend its waiting time target for NHS treatment to allow for "clinically justified" delays.
It had promised that all NHS patients in England would wait no more than 18 weeks for treatment from the point of referral by their GP by December 2008.
Latest figures show seven out of 10 patients are being treated on target, but that one in 10 wait over a year.
Health minister Ben Bradshaw said the target may apply to 90% of patients.
He said such a "clarification" of the target would act as a buffer, meaning patients were treated based on clinical need.
For example, a patient might need to lose weight before they could have surgery, or the surgery date may be inconvenient for them and they might opt to wait longer.
But he stressed that no patient would have to wait more than 18 weeks for treatment unless they chose to or it was imperative to delay treatment for clinical reasons.
The latest figures show 76% of patients who did not need to be admitted to hospital were treated in 18 weeks or less in August 2007.
This means that three out of every four non-admitted patients are seen and treated within 18 weeks following the initial referral by their GP.
Updated figures for patients admitted to hospital for treatment also showed an improvement, with 56% treated within 18 weeks from referral - up from 48% in March.
The specialty treating most patients within 18 weeks was gastroenterology, followed by general medicine and cardiothoracic surgery.
But departments like orthopaedics were struggling to meet the target.
Health minister Ben Bradshaw said: "Ten years ago it was common for people to wait 18 months for an operation.
"Since then, the NHS has come a long way and it is making good progress towards effectively ending waiting next year.
"With million of patients in the system at any one time, it is encouraging that majority of patients are already being seen within 18 weeks."
He conceded that some places were making progress faster than others, but added: "I'm confident that by the end of 2008, patients can expect to start their treatment within a maximum of 18 weeks from GP referral to treatment."
He told Radio Five Live: "The original promise was that no one would have to wait for more than 18 weeks. That has not changed.
"It is a target of 90% plus. That's the buffer zone we have given hospitals."
Dr Jonathan Fielden, chairman of the British Medical Association's Consultant's Committee, said: "The BMA has been calling for high quality and safe clinical care to be put first for patients for some time.
"'A possible 10% buffer will allow trusts to move the focus from purely finance to quality and ease some of the pressure to meet the politically set 18 week target."
Liberal Democrat shadow health secretary, Norman Lamb MP, said: "Ministers will struggle to convince a public already sceptical about government statistics that this change isn't merely providing political cover because of fears they will fail to achieve their target.
"Giving every patient an entitlement to high quality care would deliver better improvements in the NHS than a centrally-imposed bureaucratic target."