Hillingdon Hospital sits in the Uxbridge and South Ruislip constituency, where a by-election will take place on 20 July following the ex-prime minister's departure from the Commons.

Mr Streeting toured the site on Monday - and while he had praise for the staff, he compared the buildings to "a Dad's Army site".

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Speaking to reporters during the visit, the Labour MP said: "This is by far the worst hospital I have ever seen as shadow health secretary, indeed as a member of parliament.

"Staff are working in intolerable conditions, patients are sweltering in intolerable conditions on such a hot day.

"I think it's outrageous, actually, that the Conservatives have lumbered the people of Hillingdon with such an unacceptable building for so long. I mean, it resembles a Dad's Army site."

Hillingdon is one of the trusts named in the Conservative Party's list of hospitals they plan to rebuild, with date of 2030 set for its completion.

But the shadow health secretary said patients and staff had "heard it all before" - and Labour would make a firmer commitment to "deliver the state-of-the-art hospital the people of Hillingdon need".

Mr Streeting added: "Look, I don't think there is a better monument to 13 years of Conservative failure than this absolute monstrosity of a hospital building behind me in the constituency of the former Conservative prime minister.

"It is the worst hospital I've ever been to. I feel desperately sorry for the staff who have to work, and I feel even more sorry for the patients that are sweltering in it.

"And you know what the Conservatives said? Oh, 'we're going to build a new one when after the next general election'. We've heard it all before.

"I'm making a commitment today that the next Labour government will deliver the modern state-of-the-art hospital that the people of Hillingdon need. And we will do it in the first term of a Labour government."

In their 2019 manifesto, the Conservatives pledged to build 40 new hospitals by 2030 - but the scheme has come under increasing scrutiny when it comes to costs, completions dates and even the definition.

For example, some trusts are only expecting either new wings to existing hospitals, a new clinical building on an existing site, or major refurbishments rather than a new build.

In May, Health Secretary Steve Barclay insisted the government would meet its target, despite having to add five new sites to the programme over safety fears.

But that meant seven of the original sites may not have work completed by the end of the decade.

Health spokesperson for the Liberal Democrats, Daisy Cooper, said: "Hospitals across the country are falling apart, from roofs at risk of collapse to sewage leaks in operating theatres.

"Hillingdon was promised a new hospital by 2030, but it doesn't even have planning permission yet. Yet another Conservative broken promise on the NHS, and this time in Boris Johnson's backyard."

Sky News has contacted the Department of Health and Social Care for a response.