Theresa May’s pledge of £8bn more for the NHS is far less than the service needs and could damage the quality of care and force it to reduce patients’ access to treatment, experts have said.

While the minimum £8bn extra over the course of the next parliament will give the NHS more money than planned between 2018 and 2020, it will still leave it facing what health finance specialists called a £12bn hole in its finances by 2020-21.

Health thinktanks said the prime minister’s extra investment was well below the much larger sums that the House of Lords and the government’s own Office for Budget Responsibility have recently said the NHS needs given the growing pressures it is under.

“Under the Conservatives’ manifesto plans, healthcare funding would not match the demand and cost pressures on the health service, which the independent OBR estimates at more than 4% a year above inflation,” said Prof Anita Charlesworth, director of research and economics at the Health Foundation and former director of public spending at the Treasury.

“A projected £12bn funding gap by 2020-21 would require the NHS to continue to deliver major efficiency savings if quality and access to services are to be protected.”

But, she added, the NHS would find it “very challenging” to deliver the 3% a year efficiency gains needed to plug the gap, given the ageing and growing population, chronic understaffing across the service and demand for new medicines.

All three main parties’ funding pledges would leave the NHS short of enough money to do its job properly, Charlesworth added. Labour’s promise of £37bn extra over five years would leave the NHS facing a £7bn budget hole by 2020-21, while the Liberal Democrats’ plan to put an extra £6bn a year into health and social care would mean the gap was £9bn by then, and the Tories £12bn, the Health Foundation estimates.

“Whoever wins on 8 June, the next five years will see a continuing gap between the money available for the health service and the demands placed on it,” she said.

Despite rising demand for NHS care the Conservatives’ plan would mean that it received an even smaller share of national income in future years – down from 7.3% to 7% – said Nigel Edwards, chief executive of the Nuffield Trust.

“Today’s announcement means a shrinking share of national income going to the NHS, and is far below the spending increases projected by the OBR. The reality is that coming after a decade of austerity these plans are likely to mean no end in sight to the service’s struggles to deliver enough care, on time, within budget. Patients will not get the standards of treatment the NHS is capable of providing,” Edwards added.

Conservative sources confirmed that the “minimum of £8bn in real terms over the next five years” included about £2bn of the extra £8bn extra investment in the NHS over five years which George Osborne, the then chancellor, announced during the 2015 general election campaign.

May’s plan will see the NHS budget in England grow from £123.7bn this year to at least £131.7bn by 2022-23.

The Tory manifesto also committed the party to another significant increase in NHS funding, namely “the most ambitious programme of investment in buildings and technology the NHS has ever seen”. While the details of the planned five-year capital spending schedule will not be announced until the autumn, sources said that it would involve further extra “billions” being ploughed into the NHS’s physical infrastructure.

The manifesto says that that money will “enable more care to be delivered closer to home, by building and upgrading primary care facilities, mental health clinics and hospitals in every part of England”.