Child poverty is at risk of rising to a record 60-year high under a Conservative government because its manifesto retains the coalition’s benefit cuts, according to the Resolution Foundation thinktank.

An analysis by the organisation predicted a rise in the number of children living in relative poverty under a Boris Johnson-led government to 34.5% in 2023-24 up from 29.6% in 2017-18.

The thinktank released its research after the manifesto launches showed a huge gulf in what the two main parties are prepared to put into public spending, with Labour committing 28 times as much as the Conservatives.

Johnson set aside an extra £2.9bn a year by the end of the parliament that will largely go into more nurses, GP appointments and free childcare, while Jeremy Corbyn set out an extra £83bn a year for a programme of free broadband, scrapping university fees, reversing benefit cuts and extra funding for the NHS and social care.

The Resolution Foundation said Labour’s £9bn of extra spending on social security would mean 550,000 fewer children in poverty but would not lead to current poverty rates falling.

The thinktank said major policy changes since the last Labour government in 2010 have resulted in overall spending on social security being £34bn lower than it would otherwise have been.

Labour has said it would aim to end poverty altogether and has committed to ending the Conservatives’ two-child limit on benefits, as well as scrapping the bedroom tax, the benefit cap and universal credit.

The party has repeatedly said the benefits freeze would end. However, the thinktank said there were no specific promises from Labour to reverse the effective cuts already established by the failure to increase payments.

John McDonnell, the shadow chancellor, said the Resolution Foundation rightly acknowledged its £9bn of increases to benefits but “fails to take account of Labour’s plans to tackle the root causes of child poverty.” He said they included free school meals for all primary school children, expansion of free childcare, guaranteeing a right to food, improving pay and workers’ rights and tackling the housing crisis.

The Resolution Foundation said the rise in relative poverty expected under a Conservative government was largely because of the impact of the two-child limit on support for families, which is mostly still to take effect.

Laura Gardiner, the research director at the Resolution Foundation, said: “Against the backdrop of major cuts, the parties’ manifestos do offer big choices on social security.

“Under the Conservatives little is set to change, and child poverty risks reaching a record high in the coming years. Labour and Liberal Democrat pledges to spend £9bn more would mean child poverty being over 500,000 lower than under Conservative plans. However, this would not do enough to see child poverty fall from today’s already high levels.”

A Conservative spokesman defended the party’s actions on tackling poverty. “We are committed to tackling child poverty and have made progress since we came into government – with 730,000 fewer children in workless households,” he said.

“But we know that we must continue to make every effort on this issue and our manifesto sets out how we will use the tax and benefits system to do this. The prime minister has committed to giving every child in the country the opportunities to make the most of their talents.”

Labour, in its manifesto, included a promise to “end poverty by guaranteeing a minimum standard of living”.

With just over two weeks to go until election day, the Conservatives are still ahead in the polls but Labour strategists believe there is still time to close the gap by highlighting the party’s plans to put tens of billions into public services compared with the comparatively tiny amount offered by Johnson’s manifesto.

Speaking to the Guardian, Corbyn defended the £83bn-a-year level of spending on public services as bringing the UK into line with other industrial countries.

“At the end of all of our proposals – every single one carried out – we will still be spending less on public services than France or Germany. We won’t even be at their levels. It will move us into the middle ranking of the spending of industrial countries,” the Labour leader said during a campaign event at a college in Kirkby-in-Ashfield in Nottinghamshire.

Corbyn’s promised to overturn austerity with £83bn of spending include £5bn each for schools and early years and £10.8bn on social care spending. The party is also pledging payouts for women affected by the rise in the state pension age with a £58bn spending commitment over five years.

“We cannot go on underfunding education and not see the price. We cannot go on not investing in housing and not see the price. We cannot go on underfunding health and social care and not see the price,” said Corbyn.

“Yes people are being strained by the price of austerity through poverty and homelessness. The middle class, middle aged also pay it because they are having to support their kids through university.

“But if we care as a society, an inclusive society we have to be prepared to invest to achieve.”

According to data collected by the Organisation for Economic Co-operation and Development (OECD) France, Italy, Sweden, Greece and Germany all dedicate more of their GDP to public spending on social goods than the UK at rates of between 25% and 31%. This includes investment in health, old age, incapacity-related benefits, family, work programmes, unemployment, and housing.

The UK spends 21%, which is slightly more than the US, Australia at 19%, according to worldwide data collected in 2016.

Corbyn denied that anyone earning less than £80,000 a year would be taxed any more under a Labour majority government. This is despite it emerging that the party plans to remove the £250 marriage tax allowance for couples earning under £50,000.

Asked if people earning under £80,000 will pay more tax, he said: “No, not at all. The tax rates will go up for those in the top 5%. Corporation tax will go up. Inheritance rate will change, we’ve made that very, very, clear. The majority … 95% of the population will pay no more.

“We have a seriously costed manifesto. You can’t do it without raising the money from somewhere and we think it’s only right to do that.”

Corbyn said the money Labour has pledged to spend on helping women born between 1950 and 1960 who were affected by the dramatic state pension age rise and had their retirement date delayed will be paid for from “contingencies”.

He said: “It is obviously additional. We take it from government contingencies over a tapered period. But I think it’s a moral issue. The Waspi [Women Against State Pension Inequality] women were shortchanged both in the 90s when the legislation first went through but particularly in 2011 when there was this massive speed up of it they didn’t know about.”

The Institute for Fiscal Studies has described Labour’s tax plans as “not credible”. However, Labour’s plans for a £10 an hour minimum wage, a national investment bank, a green industrial strategy, and an extra £80bn a year of borrowing to fund infrastructure projects was on Monday backed by 163 economists.

In a letter to the Financial Times, a paper that has been critical of Labour’s radical manifesto, the group of left-leaning economists say the party’s approach was justified by Britain’s array of structural problems, including a lost decade for productivity growth, a dearth of investment and the gulf between London and the south east and the rest of the country.

“As economists, and people who work in various fields of economic policy, we have looked closely at the economic prospectuses of the political parties”, the letter said. “It seems clear to us that the Labour party has not only understood the deep problems we face, but has devised serious proposals for dealing with them. We believe it deserves to form the next government.”