Boris Johnson’s national insurance hike to fund health and social care will hit NHS and care workers with a GBP900m tax bill, the Guardian can reveal.

The prime minister announced this month that contributions are to increase by 1.25 percentage points to tackle the NHS backlog and pay for social care changes.

Now an official analysis of the impact of the tax rise, performed by the House of Commons library, shows it will inflict a “terrible” financial toll on health and social care workers and their families.

The report reveals they will pay 12% of the GBP7.4bn expected to be raised from employees through the tax rise. Nurses, care home staff and other health and social care workers will pay an additional GBP900m in tax, according to the analysis.

The figures do not include those working in the NHS or care who are self-employed, meaning the real impact is likely to be even greater.

Millions of health and care workers are already facing a squeeze on their finances. A combination of rising energy and consumer goods prices, coupled with benefits cuts, are adding hundreds of pounds in costs for households, charities have warned.

The Bank of England said on Thursday that inflation was expected to rise in the short term, projecting it would move “slightly above 4%” in the fourth quarter of 2021.

The Liberal Democrats, which commissioned the analysis, said the findings exposed the fact the government was launching a “tax raid on NHS and care home heroes” who are still risking their lives on the frontline of the pandemic.

A nurse or midwife on an average salary will see their tax bill soar by GBP310, care home workers will have to pay at least GBP140 more and ambulance staff will be hit with a GBP420 increase. On average, 1.4 million NHS staff will each have to fork out GBP315 more a year, the Commons library report says.

Munira Wilson MP, the Liberal Democrats health spokesperson, said: “Boris Johnson’s broken manifesto promise amounts to a GBP900m tax raid on our NHS and care home heroes.

“These are the nurses, ambulance drivers and care workers who worked day in day out to save lives on the frontline of the pandemic. Now their reward is to be hit with a tax bill worth hundreds of pounds, at a time when incomes are being squeezed by the cost of living crisis.

“The Conservatives must rethink this unjust tax hike that will disproportionately impact those on low and middle incomes. Their plans risk worsening the staffing crisis in our care homes instead of fixing it.”

Christina McAnea, general secretary of Unison, Britain’s biggest health union, said that punishing key workers with the national insurance rise was a terrible way to treat those “who’ve done so much to care for people and save lives these past 18 months”.

“Inflation has bypassed the NHS pay rise,” she said. “Most care employees have had nothing at all. The harsh universal credit snatch has yet to take effect. And these same key workers will be paying through the nose when the [national insurance contributions] increase hits.”

McAnea said the NHS and social care sector could see thousands quit as a result. “No one could blame care and NHS staff for jumping ship for more lucrative, less stressful jobs,” she said. “But the consequences of losing thousands of experienced workers simply do not bear thinking about.”

Responding to the Commons library analysis, the Royal College of Nursing council chair, Carol Popplestone, said: “Reaching primarily into the pockets of our hardworking healthcare workers who already feel taken for granted, which could lead more to the exit, is the opposite of levelling up.

“The prime minister needs to come up with a credible plan to make up for years of underinvestment and put money into the services patients will rely on for years to come, and the people who provide them.”

The government has been approached for comment.