Five MPs led by Sarah Wollaston, the Conservative chair of the Commons health select committee, have demanded the government abandon its “incorrect” claim that it is putting £10bn into the NHS annual budget by the end of parliament.

Here we look at some questions around what the real figures are – and what the dispute could mean for Theresa May.

How much extra money is the government planning to give the NHS by 2020-21? Is it £10bn, £8bn, £6bn or £4.5bn – or none of those?

This is an incredibly complicated subject, and also a key issue in British politics. May says it’s £10bn, and that is the figure Jeremy Hunt, her health secretary, routinely uses. That has changed from April 2015 when, during the general election campaign, the then chancellor, George Osborne, revealed – in an article in the Guardian – that the Conservatives planned to give the NHS £8bn more above inflation by 2020-21.

A letter on Monday from five members of the health select committee directly rebuts the prime minister’s claim. They say the true amount is either £4.5bn or at most £6bn. James Davies, a Conservative MP and, like Wollaston, a qualified GP, was among the five signatories.

Why do different people come up with such wildly differing amounts?

It all depends on when you date the budget increase from, and there are a lot of politics involved, given the extra money is a proud boast for the government to make concerning an institution the public cherishes.

When Osborne pledged the extra £8bn, he intended that money to arrive in the five years between 2016-17 and 2020-21. However, soon after the Tories won the election, the government began routinely referring to the extra money it was giving the health service as £10bn. Osborne had originally not included the £2bn additional resources he had allocated to the NHS for 2015-16 in his 2014 autumn statement.

Osborne delivers his autumn statement in 2014. Photograph: PA

The £8bn was going to be on top of that, and that amount – £8bn – was due to be the sum total of the extra money the NHS would receive. But ministers – including May – have added the £2bn to the £8bn in a way that was never intended, to find the bigger £10bn figure that the MPs have contested.

What do Wollaston and the other MPs say about the £10bn claim?

The MPs’ letter to Philip Hammond, the chancellor, says: “The continued use of the figure of £10bn for the additional health spending up to 2020-21 is not only incorrect but risks giving a false impression that the NHS is awash with cash. This figure is often combined with a claim that the government ‘has given the NHS what it asked for’. Again this claim does not stand up to scrutiny as NHS England spending cannot be seen in isolation from other key areas of health spending.”

The MPs add: “The £10bn figure can only be reached by adding an extra year to the spending review period [covering 2016-17 to 2020-21], changing the date from which the real-terms increase is calculated and disregarding the total health budget.”

Much of the extra funding for the NHS is coming from cutting £3.5bn from other parts of the health budget, notably public health and medical education and training.

“The overall impact is that total health spending – the Department of Health’s budget – will increase in real terms, at 2015-16 prices, by £6bn between 2014-15 and 2020-21. If the spending review period is considered – 2015-16 to 2020-21 – that increase is £4.5bn,” the MPs say.

The three main health thinktanks – the King’s Fund, Nuffield Trust and Health Foundation – agree that £4.5bn is the extra amount the NHS will be receiving by 2020-21, not £10bn or £8bn.

What has Theresa May said about this?

The prime minister told the Manchester Evening News on 17 September: “[The NHS England chief executive] Simon Stevens was asked to come forward with a five-year plan for the NHS. He did that, so that’s been generated by the NHS itself. He said that it needed £8bn extra – the government has not just given him £8bn extra, we’ve given him £10bn extra. As I say, we have given the NHS more than the extra money they said they wanted for their five-year plan.”

May repeated the same claim at prime minister’s questions in the Commons two days later. However, Stevens publicly disagreed with May the day after her MEN interview, when he gave evidence to the health select committee.

Stevens has welcomed the fact that Osborne’s £8bn extra was “frontloaded” to give the NHS a £3.8bn boost to its budget this year, 2016-17. But he told the select committee that contrary to what May and Hunt say, the planned phasing of the rest of the £8bn means that the NHS is not due to get the money it needs between 2017-18 and 2019-2020.

Simon Stevens, CEO of NHS England, who publicly disagreed with May. Photograph: Jonathan Brady/PA

Per capita funding for health is due to be flat next year and then to fall in 2018-19, despite the claims of £8bn/£10bn extra. The five MPs argue that this is unwise and unsustainable, given demand for healthcare is growing at 3%-4% a year as a result of the ageing and growing population and rising prevalence of lifestyle-related conditions.

And is the £8bn/£10bn the extra money the NHS asked for?

No. When Stevens launched the NHS Five Year Forward View in October 2014 he made clear that £8bn was the bare minimum the health service needed to keep functioning well, improving the quality of care and also transforming how it operated to make itself sustainable. The £8bn would still leave the NHS having to find £22bn of efficiency savings to close the £30bn gap in its finances that it expected to emerge by 2020-21 unless remedial action was taken.

What will happen now with the £10bn claim?

It will be interesting to see whether May and Hunt keep using the £10bn figure, given the widespread doubt about its accuracy. The MPs’ letter may give Jeremy Corbyn, the Labour leader, some ammunition for PMQs. Wollaston has asked Hammond for a meeting to discuss funding of health and social care before he delivers his first autumn statement on 23 November.

The chancellor is under growing pressure from NHS organisations, the Tory former health secretaries Andrew Lansley and Stephen Dorrell, and from health charities – and now the five MPs – to use that occasion to provide more money for the NHS, social care or both.