The waiting list for NHS England care could rise to 14 million next autumn if ministers do not increase its capacity, a think tank report has warned.
Health Secretary Sajid Javid warned last month that waiting lists could reach 13 million people before they begin to fall.
But the Institute for Fiscal Studies (IFS) said millions of people missed out on treatment during coronavirus and millions more weren't referred on the waiting list.
In a report published today, the IFS said: "Under this scenario, waiting lists would soar to 14 million by the autumn of 2022 and then continue to climb, as the number joining the waiting list exceeds the number being treated."
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Shadow Health Secretary Jonathan Ashworth said the report should "be a wake-up call for ministers".
It is not known how many patients who missed out on NHS care will come forward for treatment.
But analysis warned even if two thirds of "missing" patients returned with the NHS running at 95% of its pre-pandemic capacity over the next few years, waiting lists could climb to more than 15 million by the end of 2025.
The Government has highlighted it gave the NHS an extra £1billiion this year to clear its backlog.
But analysis from the Nuffield Trust has found the NHS could end its financial year with an overspend of £5billion - and this includes the extra costs of dealing with Covid.
Mr Ashworth added: “It’s unacceptable that so many patients are waiting longer in pain and distress for vital treatment including cancer care.
“Years of underfunding and cuts to healthcare had already left our NHS with ballooning waiting lists before the pandemic hit.
"Given these warnings it’s now even more urgent that a long term rescue plan is put in place to give the NHS the resources and staff it needs to bring down waiting lists and reducing waiting times for treatment.”
Chris Hopson, chief executive of NHS Providers told the Chancellor his autumn spending review will make or break the NHS.
Writing in The Observer Mr Hopson said: "Frontline leaders can’t provide the quality of care patients need, and deliver the government’s manifesto commitments, unless they are properly funded to do so.”
A Department of Health and Social Care spokesperson: “We are committed to making sure the NHS has everything it needs to continue providing excellent care to the public as we tackle the backlogs that have built up during the pandemic.
"This year alone we have already provided a further £29bn to support health and care services, including an extra £1bn to tackle the backlog.
"This is on top of our historic settlement for the NHS in 2018, which will see its budget rise by £33.9bn by 2023-24.”
