More than five million people could be stuck on NHS waiting lists within two years, leaked documents show.

The number of people waiting deemed "sustainable" by health experts is three million, but the figure already stands at 3.7 million, prompting concerns that thousands of lives could be put at risk.

That number is expected to rise to 5.5 million by 2019, according to a projection by NHS Improvement, the government body that oversees quality in NHS trusts.

Patients waiting more than four months for surgery are also expected to double by 2019, from 390,000 to 800,000.

The figures were first reported by the Health Service Journal after it acquired the leaked document from NHS Improvement.

It comes after NHS England chief executive Simon Stevens announced in March that the target for treating 92% of patients with 18 weeks would be dropped to help hospitals concentrate on other areas such as A&E and mental health.

The latest NHS England performance data, released on 13 April, showed that February saw a near-40% increase in the number of people waiting over 18 weeks to receive treatment.

Extreme pressure on NHS

Responding to the figures, a Royal College of Surgeons spokesperson said: "The performance data hammers home just how much waiting times have deteriorated in the last year.

"There are now 103,505 more patients waiting longer than 18 weeks for planned treatment than the same time last year. It also begs the question: How long will waiting lists be at this time next year given NHS England's decision to de-prioritise the 18-week waiting time target for planned surgery?

"Whether you are a patient admitted to A&E for an unexpected illness or injury, or someone waiting to have a long-term condition treated with surgery, at the end of the day all you really want is to receive treatment as soon as possible.

"The NHS is under extreme pressure, trying to manage the huge financial strain while treating more and more patients.

"While we welcome some of the initiatives NHS England announced last month to help with pressures on planned operations, de-prioritising the 18-week waiting time target for planned surgery risks undoing much of the progress the NHS has made on reducing long waiting times over the last decade."