The NHS backlog is being disproportionately shouldered by people in poorer areas, according to new research, amid a stark warning that waiting lists are likely to “grow significantly” because millions of people did not seek help during the pandemic.

Waiting lists for routine treatments have grown by 50% in the most deprived parts of England, compared with nearly 35% in the most affluent areas. Those in deprived areas were also nearly twice as likely as those in the wealthiest to wait more than a year for treatment, according an analysis by the King’s Fund.

The thinktank analysed the backlog of 5.61 million people – equivalent to almost one in every 10 people in England – who are waiting for treatments such as knee and hip replacements, cataract surgery and other common procedures.

Of patients on waiting lists in the most deprived areas, 7% have been waiting a year or more for treatment, compared with 4% of those in the most affluent areas, according to the research, which was shared with BBC Panorama and PA Media.

Jonathan Ashworth, the shadow health secretary, warned that the long waiting lists were in danger of leading to privatisation of the NHS.

“Waiting times have got so bad that you’ve got people are taking out payday loans, sometimes even remortgaging their homes because they cannot bear the pain, or the disruption to their lives, or fear they will lose their lives,” he said. “That is eroding the fundamental universal system that we created.”

He told a Labour conference fringe that it was “such a danger” because “Tory MPs are already saying that if people are opting out of the NHS and into the private sector, then why not give them a voucher or tax relief to go to the private sector? And then all of a sudden you are on to a slippery slope to an insurance system.”

A call for the government to take urgent action on waiting times was made by the King’s Fund and the social care champion, Healthwatch England, which also released a poll showing the toll on people’s physical and mental health from waiting for treatment.

Siva Anandaciva, chief analyst at the King’s Fund, said the pandemic had pushed NHS waiting lists to record levels and “laid bare” deep health inequalities.

“It is not a surprise that waits for NHS care vary across the country, but the fact that patients in deprived areas are nearly twice as likely to wait a year or more for planned treatment should be a wake-up call for a government that has committed to levelling up the country, and ring alarm bells for MPs in ‘red wall’ constituencies,” he said.

Healthwatch England’s survey of 1,600 people who were on the waiting list or had a loved one in need of treatment found that 54% said it was affecting their mental health. It set out a series of recommendations, calling for “interim support” such as physiotherapy, pain relief and mental health support and for the NHS to “reprioritise” treatment if people’s needs changed.

The picture of the waiting list’s impact was bolstered by a separate poll commissioned by the charity Independent Age, which found that half of those over 50 on the waiting list said that they were in pain daily. The survey of more than 8,000 over-50s found that 10% had been caught up in the NHS backlog. Of these, 52% waiting for an operation reported being in pain every day.

Despite an already dire situation, another leading thinktank said the NHS waiting list was likely to “grow significantly” after its analysis suggested that 7.5 million fewer people were sent for hospital care than expected during the pandemic.

The Health Foundation, which looked at figures between January 2020 and July 2021, suggested that the dip in referrals could be for a number of reasons: people could have put off seeking care during the pandemic, while others may have seen their GP “but not yet been referred due to the pressure on hospital services”.

An NHS spokesperson said the pandemic had inevitably had an impact on non-urgent care, but that staff had almost halved average waiting times for elective care over the last year: “Expert clinicians continue to prioritise patients with the greatest clinical need and hospitals should ensure that a point of contact is available to those waiting for treatment, including through patient advice and liaison services.”

They pointed to recent recent data showing that in July, patients were waiting on average 10.9 weeks to be seen, down from 19.6 weeks in the same month last year.

The spokesperson added: “Caring for more than 450,000 seriously ill Covid patients has inevitably had a knock-on effect on non-urgent care, and NHS staff have stepped up and made effective use of additional resources, helping carry out millions more tests, checks, treatments and operations this summer compared to last.”