Labour’s NHS “rescue package” will be funded by higher income tax rates, at 45p for those earning more than £80,000 a year and 50p for those bringing in over £150,000, John McDonnell, the shadow chancellor, has said.

He said the higher income tax rates would be in Labour’s manifesto, matching its pledges from the 2017 election.

Speaking at the Royal Society of Medicine, McDonnell outlined Labour’s promises to spend about £6bn more than the Conservatives on raising the NHS’s budget for day-to-day spending.

“The day-to-day spending increase will be funded by taxing the top 5% at a higher rate of income tax,” he said. “Income tax rates, national insurance and VAT will not increase for 95%. It is only the 5% we will ask to pay a little more. We will reduce the threshold for 45p rates for £80,000 and reintroduce the 50p rate for £125,000. That is our pledge.”

McDonnell said the shoulders of the wealthiest 5% were “broad enough to bear this”, adding that “our society as a whole gains from that contribution”.

He branded the last nine years of Tory-led government a “decade of decay” for the NHS.

“For eight years, the NHS budget increases have averaged just 1.4%; that says something about the value they place on caring for those in need,” he said.

“And when you sift through the spin of their most recent promises – which use cash terms figures – they are proposing spending just 3.3% extra a year, compared with our 4.3%. On capital, the NHS capital budget is lower today in real terms than 2010-11. And in recent weeks we have had the scandal of Tory promises for 40 new hospitals turning into just six hospital recalibrations.”

Speaking after McDonnell, Jonathan Ashworth, the shadow health secretary, gave further details of Labour’s NHS “rescue plan” revealed overnight.

The party’s pledge would give the health service £5.5bn more a year by 2023-24 than the £20.5bn the Conservatives have promised, and represent the biggest boost to health spending since Labour was last in power between 1997 and 2010.

After the speech, McDonnell was challenged over the impact on the NHS of the party’s plans to aim for a four-day working week, which the Tories claim would cost billions.

Earlier, Ashworth had suggested on the BBC it would not apply to the NHS, saying: “No. It is not happening. There is not a four-day week coming in the NHS.”

However, standing beside Ashworth, McDonnell clarified at the event that the policy would apply to everyone when brought into force over the course of a decade. He said: “It’s a 32-hour working week, implemented over a 10-year period. It will apply to everybody. What Jon was talking about was how you apply it over that 10-year period.

“What we’ve said – a bit like the implementation of the minimum wage – just like every other reduction in the working week, it is planned and negotiated. As we put investment into our economy, there will be higher wages and over time as wealth is created, we want a proportion of that wealth shared more fairly with the workforce. That means higher wages or of course a shorter working week.”

Labour’s spending pledge puts pressure on Boris Johnson to increase the amount he is committing to the NHS, which he has made one of his three “people’s priorities” and sought to make a Tory vote-winner.

Labour says the sums it is pledging would end the lengthening delays faced by patients for A&E care, cancer treatment and planned operations, tackle the NHS’s worsening staffing crisis, restore bursaries for student nurses, improve mental healthcare, let hospitals buy scores of CT and MRI scanners and pay for a new generation of hospitals, GP surgeries and mental health facilities.

Health thinktanks and organisations representing NHS staff welcomed Labour’s plans because they would end a decade of much smaller budget increases – of just over 1% a year until last April – since the Conservatives came to power in 2010 compared with the 3.7% it has averaged, under all governments since its creation in 1948.