Video report by ITV News Political Correspondent Angus Walker
Addressing claims from the Health Foundation that the new money is a "drop in the ocean", Boris Johnson said: "Don't forget that this is £1.8 billion of new money. It wasn't there 10 days ago."
"It's on top of the extra £34 billion we are putting into the NHS and I've said that my job is to make sure that we use the funds that go into the NHS to reduce the time you wait to see your GP, the time you wait in A&E - and that's why we are doing it today."
But Labour leader Jeremy Corbyn echoed criticisms from inside the health industry, saying the new funds are not enough to fix all the hospital issues.
He said: "It goes nowhere near paying for all the cuts that have been made over the past nine years in our NHS.
"There is a number of hospitals that have been selected for some improvement, many other hospitals have been left out of it."
However Health Secretary Matt Hancock, speaking to ITV News, said the new money announced by Boris Johnson, had "come from the Treasury" and "none of this is recycled within the NHS".
He said: "There's £1.8 billion increase in the NHS capital budget - every single penny is extra money allocated from the Treasury.
"Some of that means that I can unlock investments that hospitals wanted to do. In other cases I'm able to announce new investments that have gone into some areas but every single penny of the £1.8 billion is newly allocated from the Treasury."
The Nuffield Trust chief executive appeared to disagree with Mr Hancock, telling ITV News "this money is not necessarily quite new".
Nigel Edwards said: "Trusts had acquired surplussed but had not been allowed to spend it so part of this so-called extra money is in fact releasing money they had already got, some of it is new."
The health secretary also claimed the 20 NHS hospitals selected to gain from the spending pledge were chosen because they had the "best" improvements planned, - not because they had cut their costs.
But Shadow Health Secretary Jon Ashworth claimed the hospitals chosen had been "promised" money for cutting their costs and after being blocked from spending, had now been told "they can spend that extra money after all".
Mr Ashworth, speaking on BBC Radio 4's Today Programme, echoed that point: "What it is beginning to look like is that money hospitals were promised for cutting their costs and cutting back on their spending, they were promised extra money, and then ministers came along and blocked them from spending this extra money."
He added: "It now looks like all Boris Johnson has actually done is said they can spend that extra money after all. So, there is huge scepticism about whether it is new money."
Regardless of where it came from, experts have said the sum, while desperately needed, is just a fraction of what is required to fix ailing NHS buildings across the country.
Ben Gershlick, from the Health Foundation charity, said that “years of under-investment in the NHS’s infrastructure means this extra money risks being little more than a drop in the ocean”.
He warned that NHS facilities are “in major disrepair” in England, with a maintenance backlog of more than £6 billion, a figure also cited by other experts.
Labour's Jon Ashworth claimed the cash "falls significantly short" of what some hospitals need to make essential repairs.
He told ITV News: "Hospitals have ceilings collapsing, we have sewage pipes bursting, we've got cancer patients having their diagnosis appointments cancelled because the equipment is faulty.
He added: "This announcement – even if it’s ever delivered – falls significantly short of what’s needed to provide quality, safe care to patients after years of Tory cuts."
Speaking about how the new investment would help staff shortages in the NHS, Mr Johnson said: "That's why it's so vital we are putting another £1.8 billion of new money, £800 million of that going on various projects, a billion of capital, but that's on top of the £34 billion we are putting into the NHS.
"But of course, the lion's share of that will go on staffing - on attracting, retaining and professionally developing the staff in the NHS.
"I've made it my mission to ensure that people across the country do get to see their GP in time, do get through A&E in a reasonable time, and that's why it's vital to invest now."
Here is a list of the beneficiaries, and what funding they will receive:
East of England
Luton and Dunstable University Hospital NHS Foundation Trust (FT): £99.5 million for a new block in Luton to provide critical and intensive care, as well as a delivery suite and operating theatres
Norfolk and Norwich University Hospitals NHS FT: £69.7 million to provide diagnostic and assessment centres in Norwich, Great Yarmouth and King’s Lynn
Norfolk and Suffolk NHS FT: £40 million to build four new hospital wards in Norwich, providing 80 beds
NHS South Norfolk Clinical Commissioning Group: £25.2 million to develop and improve primary care services
University Hospitals Birmingham: £97.1 million to provide a new purpose-built hospital facility, replacing outdated outpatient, treatment and diagnostic accommodation
United Lincolnshire Hospitals Trust: £21.3 million to improve patient flow in Boston by developing urgent and emergency care zones in A&E
Wye Valley NHS Trust: £23.6 million to provide new hospital wards in Hereford, providing 72 beds
University Hospitals of North Midlands NHS Trust: £17.6 million to create three new modern wards to improve capacity and patient flow in Stoke, delivering approximately 84 beds for this winter
Barking and Dagenham, Havering and Redbridge CCGs and North East London NHS Foundation Trust: £17 million to develop a new health and wellbeing hub
Croydon Health Services NHS Trust: £12.7 million to extend and refurbish critical care units at the Croydon University Hospital
North East and Yorkshire
South Yorkshire and Bassetlaw Integrated Care System: £57.5 million for primary care investment
Newcastle upon Tyne Hospitals NHS FT: £41.7 million to improve paediatric cardiac services in the North East
Leeds Teaching Hospitals NHS Trust: £12 million to provide a single laboratory information management system across West Yorkshire and Harrogate, covering all pathology disciplines
Greater Manchester Mental Health NHS FT: £72.3 million to build a new adult mental health inpatient unit
Mersey Care NHS FT: £33 million to provide a new 40-bed low secure unit for people with learning disabilities
Stockport NHS FT: £30.6 million to provide a new emergency care campus development at Stepping Hill Hospital in Stockport, incorporating an urgent treatment centre, GP assessment unit and planned investigation unit
NHS Wirral CCG: £18 million to improve patient flow by improving access via the urgent treatment centre
Tameside and Glossop Integrated Care NHS FT: £16.3 million to provide emergency and urgent care facilities at Tameside General Hospital
Isle of Wight NHS Trust: £48 million to redesign acute services
Royal Cornwall Hospitals NHS Trust: £99.9 million to build a new women’s and children’s hospital in the centre of the Royal Cornwall Hospital site in Truro