A deliberate slowdown in the treatment of people with less serious ailments and the discovery of groups of patients who were not previously recorded in national statistics could be behind growing waiting lists, experts have said.

The latest NHS England data on 18-week referral times for elective operations shows an additional 260,000 people were waiting for a procedure in September compared to the same point last year.

This year’s figures show the usual seasonal variation in elective waiting lists, with the total rising over the summer and falling after August, but levels are far higher. In August and September, there were 2.94 million and 2.9 million waiting respectively − the highest two monthly totals recorded since April 2008.

Compared to the first six months of previous years, the hospital sector is performing more procedures on admitted patients, whose conditions are often more serious.

The NHS performed 1.84 million elective procedures on admitted patients in the first half of 2013-14 - 62,000 more than in the same period in 2012-13. However, the 5.01 million procedures it performed on non-admitted patients was 156,000 fewer than the previous year.

The median wait for non-admitted patients is 5.2 weeks so far this year − approximately one week higher than the same period in the preceding five years, across which the median varied only slightly between 4.1 and 4.4 weeks.

Waiting times experts suggested trusts may have allowed non-admitted lists to grow as they focused on more acute patients.

Nigel Coomber, director of the NHS Interim Management and Support elective care intensive support team, said he thought the health service would overall hit its waiting times targets for 2013-14, but more individual trusts would breach them than last year.

He said there was no single cause: “Sometimes it’s a capacity and demand mismatch. Sometimes they have just built up a backlog of patients and once that happens it can be very hard to claw back.”

Mr Coomber added that several trusts had identified groups of patients who had been referred but not previously recorded in waiting times statistics, and that improved IT systems had led to greater numbers of patients being recorded in the referral to treatment time data.

Barnet and Chase Farm Hospitals Trust in north London declared a serious incident this week after it confirmed to HSJ that problems with its computer system had seen patients facing “unacceptable delays in treatment”. The trust is responsible for 242 of the 386 patients listed in the latest national figures who had been waiting more than a year for treatment.

Earlier this month, North West London Hospitals Trust announced it had failed to properly record the waiting times of 60 per cent of its elective inpatients, and had failed to start the clock when 2,700 people on its inpatient waiting list were referred for treatment.

North London’s Whittington Health Trust suspended its reporting of waiting times this month after problems with a new McKesson IT system.

NHS England said part of the fall in non-admitted patients treated was down to responsibility for commissioning consultant-led sexual health services moving to local authorities, because these patients no longer showed up in NHS data.

HSJ columnist Rob Findlay, director of waiting times software firm Gooroo, said: “Why has the NHS not been keeping up with demand? Partly because of the financial squeeze, perhaps. There is a widespread practice of rolling this year’s activity into next year’s plan. So demand and activity drift apart and you get waiting list growth and target breaches.”

Barry Mulholland, a consultant for MBI Health, said trusts were hindered by not understanding reporting rules, a high turnover of clinical administrative staff and failure to manage clinical variation and behaviour.