NHS and social care workers in England will pay an estimated £390m extra to subsidise their own services when the National Insurance rise comes into effect in April, exclusive analysis by openDemocracy has revealed.

Nurses will take a £275-a-year pay cut on average as a result of the rise – despite Boris Johnson saying the money was needed to “pay good wages for the 50,000 nurses” that he pledged to recruit by 2025 when the plans were announced in September.

Social care workers will take an average cut of £129 a year.

The National Insurance rise will fund ‘the NHS and equivalent bodies’ and will be replaced by a formal ‘health and social care levy’ of the same value from 2023.

“Our crumbling social care system desperately needs more cash – we all know that. But a regressive tax which takes money from the pockets of the lowest-paid workers is not the way to do it,” said Rachel Harrison, national officer for the GMB trade union, which represents NHS and care workers.

“The Conservatives’ National Insurance hike will snatch hundreds of millions from the pockets of carers and NHS workers. It's a completely backwards way of raising the money and it will ultimately be self-defeating.”

The lowest-paid NHS and social care staff – those earning £24,000 or less – will lose £66m in total from their pay cheques, calling into question the prime minister’s promise that the money “will go straight to the front line” of the health service.

Of that figure, £47m will come from the lowest-paid NHS staff and £19m from the lowest-paid social care staff.

The government has said that the manifesto-breaking tax rise, which is expected to raise £12.4bn a year, will be spent on tackling the backlog of patients waiting for treatments caused by the pandemic and fixing long-term problems with social care.

NHS workers will lose £327m while workers in the social care sector will lose £61m in total.

Care workers will lose £4.4m a year from their pay, despite 71% already being paid below the real living wage of £9.50 in the UK and £10.85 in London, according to Skills for Care.

NHS staff in England have suffered real-term salary falls of up to £2,949 over the past decade, according to analysis by the Health Foundation published last year.

Last year, English NHS staff received a 3% pay rise following an independent pay review. However, junior doctors will lose on average £193 from their annual pay after the tax rise.

“Over the last decade we’ve seen doctors hit by repeated real-terms pay cuts, while they have been asked to work harder and go above and beyond during the pandemic,” a British Medical Association spokesperson said.

“Upcoming tax changes will further erode doctors’ take-home pay, which is why we are calling for a pay increase that reflects cost of living increases.”

The 1.25 percentage points rise in National Insurance comes as households are already struggling with a worsening cost of living crisis.

Inflation is already at its highest level for three decades and the Bank of England expects it will peak at 7% when the increase comes into effect in April.

Last month, 14 unions, representing 1.2 million health staff in England, called on the government to raise NHS pay and raised fears of a “growing exodus of exhausted staff”.

The NHS said that nursing staff shortages are its "most urgent challenge" in its 2019 recruitment plan. One in eight nursing posts and around one in 11 care worker roles are currently unfilled, with demand for the positions set to rise in line with the UK's ageing population.

The health service’s staffing shortfall of 100,000 could reach a quarter of a million by the end of the next decade, according to research by the King’s Fund think tank.

openDemocracy estimated the figures using the latest NHS Digital data on the mean full-time equivalent (FTE) annual pay of staff by job title, cross-referenced with the most recent data on the number of FTE staff in each role.

Social care statistics were estimated using data from Skills For Care’s workforce estimates by cross-referencing the number of public, private and independent FTE jobs in the social care sector against Skills For Care’s estimates for mean FTE annual pay.

The National Insurance increases were estimated by running mean FTE salaries through Which?’s National Insurance calculator for 2021-22 and 2022-23.

A Treasury spokesperson said: “We’ve taken decisive and historic action, with our Health and Social Care Levy due to raise around £13bn a year for the NHS and social care. It is a progressive tax with those earning more paying more. We rightly funded a 3% pay-rise for NHS staff this year, increasing nurses pay by £1,000 a year on average, and have committed to pay rises next year.

The Health and Social Care Levy will benefit people up and down the country, including by tackling the backlog that the pandemic has created on NHS operations and procedures strengthening the adult social care system so that people do not have to bear the financial risks of catastrophic care costs themselves.”