Boris Johnson has said he was “very, very surprised” to receive a fine for attending a lockdown-breaking gathering in Downing Street.

The prime minister, talking to Mumsnet founder Justine Roberts, was questioned on issues from partygate and Northern Ireland to nappies and his favourite book to read to his children.

Asked by a teacher why he did not lose his job for breaking lockdown rules, Mr Johnson said: “I apologise very much for what happened but just to remind her of what I did.

“I think if people look at the event in question it felt to me like a work event, I was there for a very short period of time in the cabinet office, at my desk, and I was very, very surprised and taken aback to get a fixed penalty notice.”

Following the release of the Sue Gray report last Wednesday, which revealed raucous lockdown-breaking parties in 2020 and 2021, Mr Johnson told parliament he took “full responsibility for everything that took place under my watch” and said the government were “humbled” and had “learned a lesson”.

Mr Johnson also told Mumsnet, the online forum for parents, he did not eat the cake he was given at the event he was fined for.

“If you’re talking satirically about that miserable event, whose picture appeared on the front pages, then no cake was consumed by me I can tell you, I can tell you that much,” he said.

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The first question asked to the PM, was: “Why should we believe anything you say when it has been proven your habitual liar?”

He seemed taken aback and said he did not agree with the “conclusion nor the premise”, adding: “People throw all sorts of accusations at me about all sorts of things, ever since I drove around on with a sign on a bus. And they have all sorts of reasons for saying that. But I think you just got to look at the record of what I deliver.”

On Northern Ireland, Mr Johnson admitted the protocol – which governs post-Brexit trade between Great Britain and Northern Ireland – “is certainly not functioning well”.

He insisted a hard border between Northern Ireland and Ireland “isn’t going to happen” and said, “all we’re trying to do is to get rid of some pretty pointless and bureaucratic checks”.

‘I’m very fast at changing nappies’

Moving away from politics, Mr Johnson spoke about having two young children with his wife, Carrie Johnson.

“I’ve changed a lot of nappies recently, I’m very fast I can tell you,” he revealed.

And he admitted: “Carrie, obviously, you know, does more than I do. I’m not gonna conceal that fact from you. She does far, far more than I do.

“But I, if you, I think if you really sat her down and interrogated her if she would, she would admit that I do quite a lot too.”

The PM appeared stumped when asked what his favourite young child’s book is as he paused then said he liked books by Dr Seuss – which he said he used to pronounce incorrectly.

His Mumsnet interview was released after a morning of Conservative MPs launching a fightback as a growing number of their colleagues publicly calling for Mr Johnson to resign, with more revealing they have handed in letters of no confidence in the PM.