The government’s official spending watchdog is to launch an inquiry into Boris Johnson’s claim that 40 new hospitals will be built by 2030, as concerns grow in Whitehall that the pledge is unaffordable and has been greatly oversold to the public.

In a move that could prove hugely embarrassing for the prime minister, the independent National Audit Office (NAO) has decided to conduct a “value for money review” into the entire scheme, which was a cornerstone of the Conservative party’s 2019 general election manifesto.

The NAO has also made clear that it is concerned at how the government still maintains that it will build 40 entirely new hospitals, when in reality many will merely be extensions or refurbishments of existing ones.

In a letter to the shadow health secretary, Wes Streeting, who had raised questions about delays and the resulting rising costs of the scheme with the NAO, its top official – Gareth Davies, the comptroller and auditor general – said he was already preparing a full value for money review.

Davies also said that he had taken “particular” note of the “implications of delay for increasing costs at this time of high inflation and the matter of whether all projects truly meet the classification of ‘new hospitals’”. Davies said he would be reporting back in 2023.

The NAO’s intervention will raise further questions about honesty and standards inside the Johnson government following the long-drawn-out Partygate controversy and a series of recent sex scandals involving male Conservative MPs.

On Friday, Johnson’s former deputy chief whip Chris Pincher was suspended from the party after he was accused of sexually assaulting two men at the Carlton Club in London. This was a week after their party had lost two byelections, both triggered by sex scandals involving Conservative MPs who had to step down.

Gareth Davies of the NAO said he would be addressing the ‘matter of whether all projects truly meet the classification of “new hospitals”.’

Many Tories fear their party is now becoming more widely distrusted on policy, having broken pledges not to raise national insurance, abandoned the “triple lock” on pension increases last year, and scaled back high-speed rail projects in the north of England.

The Conservatives promised to deliver “40 new hospitals” in their 2019 manifesto, but it has since been revealed that many of these projects are just improvement to existing sites.

Last year, it emerged that ministers had been instructing trusts to give an exaggerated impression to the public of the scale of the projects by referring to refurbishments as “new hospitals”.

A guidance document, distributed to trusts and entitled New Hospital Programme Communications Playbook, said a “new hospital” could be “a major new clinical building on an existing site or a new wing of an existing hospital, provided it contains a whole clinical service, such as maternity or children’s services; or a major refurbishment and alteration of all but building frame or main structure, delivering a significant extension to useful life which includes major or visible changes to the external structure”. Staff were told that all the schemes “must always be referred to as a new hospital”.

Last month, the BBC’s Reality Check programme emailed every NHS trust involved in the scheme, asking which of three categories their project fitted into. Of the 34 trusts which replied, only five said they were building a whole new hospital, 12 said they were building new wings and nine said they were rebuilding existing hospital buildings.

With inflation now running at over 9%, there are also growing fears within government that even some of these extensions could prove to be unaffordable. Several hospitals earmarked for building work, including centres in Leeds, Leicester and Manchester, are among those still waiting to hear what scale of work can go ahead and when.

Labour’s Wes Streeting: ‘The only place these 40 new hospitals’ currently exist is in Boris Johnson’s imagination’

Already, delays to building projects have resulted in additional costs to the taxpayer. Leeds General Infirmary estimates the cost of development for two new buildings will be £75m more than first planned due to delays to starting construction and the rising costs of building works.

Reacting to the NAO’s decision to launch a review and report back in 2023 – in the runup to the next general election – Streeting said: “The only place these 40 new hospitals’ currently exist is in Boris Johnson’s imagination. The election manifesto promise now looks to be another example of the Conservatives overpromising and underdelivering.

“Labour will get value for taxpayers’ money and ensure that every penny going to the NHS is spent wisely, providing better care for patients.”

The Liberal Democrats’ deputy leader and spokesperson on health, Daisy Cooper, said: “Before MPs break up for summer, the government must publish a clear timetable for its new hospitals programme and explain why they are failing to deliver their number one health pledge.

“If they don’t deliver on their number one health promise, it will be an ultimate betrayal.”

On its own website the government says: “Hospitals come in different shapes and sizes and each new hospital will be designed to meet the needs of the local area, staff and patients, now and in the future.”

Matthew Taylor, the chief executive of the NHS Confederation, recently cast doubt on whether many schemes would get off the ground. “The government launched these flagship new-builds with much fanfare, but NHS leaders are becoming increasingly frustrated that the money isn’t following through,” he said. “The fear now is that some of these schemes may never see the light of day.”