More patients may suffer harm and sue the NHS as waiting times for treatment continue to grow, the National Audit Office (NAO) has warned.
Around 40% of NHS compensation claims are already due to delays in treatment or diagnosis, but this could rise if people are left on long waiting lists, it said.
NHS England is currently reviewing health service targets and could scrap the current 18-week target for people to start treatment following a referral.
The target covers non-emergency operations such as hip and knee replacements or cataract removal.
In January, 86.7% of patients were seen within 18 weeks, below the 92% target, and the target has not been hit since February 2016.
The waiting list grew from 2.7 million to 4.2 million between March 2013 and November 2018, while the number waiting more than 18 weeks grew from 153,000 to 528,000.
During this period, the number of people treated each month has increased from 1.2 million to 1.3 million.
The head of NHS England Simon Stevens said in 2017 he accepted that waiting times would lengthen as hospitals focused on other areas such as cancer and mental health.
The NHS long-term plan said funds will be given over the next five years to “grow the amount of planned surgery year-on-year, to cut long waits, and reduce the waiting list”.
But the NAO study said patients could suffer harm from growing lists.
It said: “There is a risk that longer waiting times may lead to patient harm and negligence claims against the NHS.
“For many people, longer waits result in inconvenience and the discomfort associated with living with a medical condition.
“But for others their condition may deteriorate and a longer wait for treatment may cause them harm.”
The report said the NHS does not produce analysis to show the extent to which patient harm has occurred as a result of long waiting times.
“Given that 40% of clinical negligence claims are brought because of delays in diagnosis or treatment, there is a risk that longer waiting times may lead to an increasing number of future claims,” it said.
The NAO said that “while there has been no explicit policy to deprioritise elective care, incentives for achieving waiting times standards for elective care have been weakened or removed over the past few years”.
The report also pointed to the reduction in the number of hospital beds as a factor in growing waiting lists.
A lack of beds means “trusts will treat emergency and cancer patients first due to the urgent nature of the treatment”.
The report said: “We found that bed occupancy has increased in recent years and an increasing number of trusts now routinely operate with a bed occupancy rate well above 90%.
“The number of beds in the NHS has reduced by 7% (8,000) since 2010-11.
“While reducing excess beds may create efficiencies, after a certain point the capacity constraints this will introduce will impact on other resources such as staff and theatre usage.”
The report said that for the 18-week target to be met again, “significant additional investment will be required”.
It added: “We estimate that it would cost an extra £700 million to reduce the waiting list to the size last seen in March 2018, based on current trends.”
Professor Neil Mortensen, vice president of the Royal College of Surgeons, said patients would be very concerned by the report.
“These will be patients waiting in pain or discomfort, possibly unable to work or go about their daily lives, and worried that their condition may deteriorate,” he said.
“They may also need extra help from family members and can begin to feel as if they are a burden.
“The fact that 40% of NHS compensation claims are attributed to delays in treatment and diagnosis is a visible sign that reducing waiting times needs to be an absolute priority.
“While the NHS long-term plan commendably sets out to strengthen community services, which should help ease pressure on hospitals, more needs to be done to properly tackle the backlog of patients waiting for surgery.”
Prof Mortensen said the college supports pilots to test new NHS targets but it is “important to acknowledge that altering targets will not solve the underlying challenges the NHS faces”.
He added: “Surgeons firmly believe that there are now not enough beds within the NHS to efficiently deal with demand and there needs to be a commitment to increasing bed capacity, so that more patients can be seen in a reasonable timeframe.”
An NHS spokesman said: “The NAO is right to highlight improvements to NHS cancer care, which mean more people are surviving cancer than ever before, and that despite significant increases in demand, more people are getting quick tests and hospital treatment.
“As the additional funding to help deliver the NHS long-term plan becomes available from April, local health groups are being allocated the money they need to increase the amount of operations and other care they provide, to cut long waits.”
Sir Amyas Morse, head of the NAO, said the NHS had made good progress in some areas, such as increasing the number of urgent cancer referrals.
“However, there has been insufficient progress on tackling or understanding the reasons behind the increasing number of patients now waiting longer for non-urgent care,” he said.