The Institute for Fiscal Studies said that plans to cut the overall waiting list are unlikely to happen this year as the health service grapples with rising demand, strikes and staff shortages.
There are currently 7.2 million people on a waiting list for routine treatment, including over a million Londoners – the second highest regional total.
NHS England pledged to eliminate waits of more than two years for treatment by July 2022.
It said waits of longer than a year would be gone by March 2025, while waits of more than 18 months would be ended by April 2023.
The IFS report acknowledged that the NHS had made good progress on some of its waiting list targets, including a 94 per cent fall in the number of people waiting two years for treatment between February and November last year.
But it warned that the number of those waiting 18 months to two years had grown 7 per cent up to September.
The number of people waiting between 1.25 and 1.5 years increased from 86,000 in February 2022 to 115,300 in September 2022, an increase of 34 per cent, the IFS said.
This number was “effectively zero” before the pandemic, the think tank added.
The IFS said the fact that such numbers continue to grow “illustrates the broader challenge: while waiting lists are continuing to grow overall, it is not mathematically possible for the NHS to reduce the number of people waiting for all time periods.
“Instead, it can only prioritise reducing some groups, such as those waiting more than two years, while other parts of the waiting list continue to grow.”
The NHS has also committed to increasing treatment volumes to 30 per cent above pre-pandemic levels by 2024/25.
But the IFS report said this was “highly unlikely to be achieved – not least due to the lingering effects of Covid-19 and other pressures on the system.”
“As a result, waiting lists are – in our view – unlikely to start falling rapidly any time soon. Instead, our central expectation is that waiting lists will more or less flatline over the next year, and fall only gradually from mid-2024.”
In order for the waiting list to fall, the IFS said the NHS would need to achieve “a truly remarkable increase in how many patients it manages to treat”.
It would also need to the number of people joining the waiting list for treatment to remain “unexpectedly low”.
Max Warner, research economist at the IFS and an author of the report, said: “To its credit, the NHS has made real progress in its efforts to reduce the number of patients waiting a very long time for care, virtually eliminating waits for care of two years or more.
“But efforts to increase treatment volumes have so far been considerably less successful…
‘To turn things around, and deliver on the headline ambition of increasing elective activity to 30% above pre-pandemic levels by 2024/25, would require unprecedented double-digit growth in treatment volumes over the next two years.
“That would be more than three times the growth rate in the five years prior to Covid, and looks increasingly unreachable. As a result, it is likely that the waiting list will flatline rather than fall over the coming year.”
Responding to the analysis, NHS Providers chief executive Sir Julian Hartley said: “Mounting pressures on acute, ambulance, mental health and community services, such as chronic workforce shortages, could hamper efforts to cut the backlog further if left unchecked.
“A fully funded workforce plan from the government will go a long way to help with this and ensure recovery is sustainable.
“The ongoing strikes are also causing huge disruption to services, and risk undoing hard-won progress made on care backlogs. To bring this to an end, the government needs to talk to unions, urgently, about pay for this financial year.”