The NHS should expect a surge of medical negligence claims because of longer waiting lists in areas including cancer care and planned operations, Whitehall’s spending watchdog has said.

The National Audit Office said two in five compensation claims were due to delays in treatment or diagnosis, but this figure could rise if patients continued to be left on long waiting lists.

“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,” the independent body said in a report.

NHS England is 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.

Friday’s report points out that in January, 86.7% of patients were seen within 18 weeks, well below the 92% target which 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 rose from 153,000 to 528,000. During this period, the number of people treated each month increased from 1.2 million to 1.3 million.

The head of NHS England, Simon Stevens, has said he accepts waiting times will lengthen as hospitals focus on other areas, such as cancer and mental health.

The NAO study stated: “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 found waiting-time standards were getting worse for elective (non-urgent care) and cancer treatment, and the waiting list for elective care continued to grow.

Standards for cancer care were introduced to help improve early diagnosis and survival rates – most of these standards were met until 2017-18.

A key standard – that 85% of patients should be treated within 62 days of an urgent GP referral for suspected cancer – had not been met, since the end of 2013, auditors disclosed.

In November 2018, only 38% of NHS trusts met this standard, and between July and September 2018, 78.6% of patients were treated within this timescale.

Meg Hillier, the chair of the public accounts committee, said the NHS had failed to meet its referral waiting time target for patients needing non-urgent treatment for more than three years.

“The link between hospitals’ waiting times performance for non-urgent and cancer care and strained capacity is clear,” Hillier said. “The NHS needs to identify how it will improve waiting times as a matter of urgency.”

An NHS spokesman said: “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.”