Boris Johnson took a reporter's phone and put it in his pocket after refusing to look at a photo of a child who had to sleep on a hospital floor.
The Prime Minister was being interviewed by ITV News political correspondent Joe Pike, who asked Mr Johnson to look at a photo of four-year-old Jack, whose mother Sarah Williment covered him with coats to keep warm as he waited for a bed at Leeds General Infirmary.
She had taken him there last Tuesday fearing he had pneumonia.
Mr Johnson did not look down at the photo on Mr Pike's phone, instead saying he would "study it later" as he attempted to steer the conversation on to Tory investment in the NHS.
In a clip of the interview posted on Twitter, Mr Pike said to Mr Johnson: "You refuse to look at the photo. You've taken my phone and put it in your pocket Prime Minister."
Mr Johnson then took the phone out of his pocket, looked at the photo on the screen, and said: "It's a terrible, terrible photo. And I apologise obviously to the families and all those who have terrible experiences in the NHS.
"But what we are doing is supporting the NHS, and on the whole I think patients in the NHS have a much, much better experience than this poor kid has had.
"That's why we're making huge investments into the NHS, and we can only do it if we get Parliament going, if we unblock the current deadlock, and we move forward."
At the end of the interview, Mr Johnson said: "I'm sorry to have taken your phone. There you go."
Shadow health secretary Jonathan Ashworth said: "Refusing to even look at an image of a child suffering because of Conservative cuts to the NHS is a new low for Boris Johnson. It's clear he could not care less.
"Don't give this disgrace of a man five more years of driving our NHS into the ground. Sick toddlers like Jack deserve so much better."
Earlier, Mr Johnson apologised to "everybody who has a bad experience" in the NHS after the story of four-year-old Jack emerged.
Ms Williment told the Daily Mirror her son was eventually moved to a ward, where he waited for five hours on a trolley before a bed was found at 3am.
Diagnosed with flu and tonsillitis, Jack was allowed to be taken home at lunchtime.
Ms Williment, 34, told the Mirror she would now switch allegiance and vote Labour in Thursday's election, owing to her concerns about the state of the NHS.
She said: "I am frustrated about the system and the lack of beds, which I am presuming is due to a lack of funding to the NHS to deliver the services that are required."
Asked about the incident during an interview on LBC, the Prime Minister said: "Of course I sympathise very much and I apologise to everybody who has a bad experience.
"By and large I think the NHS do an amazing job and I think that they deserve all praise for the service they provide – but they do need investment and that's why we're doing it now.
"But they need investment from a one nation government that really cares and understands – that's us that cares and understands – and you need long-term funding."