Boris Johnson has said he is "glad" the father of a sick child challenged him about NHS funding in a hospital corridor.
The prime minister was confronted by Omar Salem at Whipps Cross University Hospital in north-east London.
Mr Salem, who said his seven-day-old daughter had been "gravely ill", told Mr Johnson there were not enough staff.
The PM later wrote on Twitter that the encounter was not "an embarrassment" but "part of my job".
He added that it did not matter whether people "agree with me" - a reference to Mr Salem's work as a Labour activist.
In a conversation lasting around two minutes, the new father said the situation he had experienced at Whipps Cross was "not acceptable".
"There are not enough people on this ward, there are not enough doctors, there's not enough nurses, it's not well organised enough," he told Mr Johnson.
"The NHS has been destroyed... and now you come here for a press opportunity."
Mr Johnson said "there's no press here" but Mr Salem gestured to cameras filming the confrontation, and said: "What do you mean there's no press here? Who are these people?"
The prime minister explained he was "here to find out" about the situation, but the man said: "It's a bit late, isn't it? Years and years and years of the NHS being destroyed."
'Part of my job'
In a post on Twitter, Mr Johnson said later that it was part of his job to talk to people on the ground.
"I've been PM for 57 days, part of my job is to talk to people on the ground and listen to what they tell me about the big problems. It doesn't matter if they agree with me," he wrote.
I’ve been PM for 57 days, part of my job is to talk to people on the ground and listen to what they tell me about the big problems. It doesn’t matter if they agree with me.— Boris Johnson (@BorisJohnson) September 18, 2019
I’m glad this gentleman told me his problems. This isn’t an embarrassment this is part of my job. https://t.co/j60ODrROXi
"I'm glad this gentleman told me his problems. This isn't an embarrassment, this is part of my job."
Mr Salem wrote in a tweet: "Boris Johnson had the temerity to come to [Whipps Cross Hospital] for a press opportunity on the children's ward that my seven-day-old daughter is on, having been admitted to A&E yesterday gravely ill.
"The A&E team were great but she then went for hours on the ward without seeing a doctor.
"Boris Johnson has been an MP, [Mayor of London], cabinet minister and now PM while the NHS has been neglected, just as my daughter was last night.
"Rather than drips of money for press opportunities he should get on with properly supporting the NHS so that patients get the care they deserve, there is adequate staffing with good working conditions and worried fathers like me can have some peace of mind."
Alan Gurney, chief executive of Whipps Cross hospital, said: "We are constantly reviewing staffing levels on our wards to ensure our patients are safe at all times, but occasionally - as in fact happened on this ward last night- an unexpected emergency in one part of the hospital can cause a temporary pressure elsewhere."
It is not the first time Mr Johnson has experienced an impassioned exchange with a member of the public since becoming prime minister in July.
On a visit to Yorkshire two weeks ago, the prime minister was told by a member of the public he should be in Brussels negotiating with the EU instead of touring Morley high street, near Leeds.