The Prime Minister grimaced and exclaimed 'you're joking' when he was shown a body scan image of the drugs package which Zachary Bowler 'plugged' inside himself.
Footage of Mr Johnson looking baffled went viral when he was shown the image on a state-of-the-art scanner during a visit to the Leeds jail in August last year.
The image was taken two months earlier when Bowler had been sent to prison after being given a jail sentence for beating his girlfriend up with a cricket bat.
Leeds Crown Court heard how prison officers spotted an "oval package" inside Bowler's body when he was scanned as he was admitted to the Victorian jail.
Stephen Welch, prosecuting, said Bowler was placed in an isolation cell and asked to retrieve the item.
The defendant handed over a Kinder Egg which contained ten buprenorphine tablets.
The class C drug is more commonly known as subutex and is often prescribed to people withdrawing from heroin.
Bowler, from Boston Spa, was interviewed and admitted hiding the Kinder egg inside his body.
He said the drugs were for his own use as he was a heroin user and was worried about withdrawal symptoms while in custody.
Bowler, 27, pleaded guilty to conveying a list A article into prison.
Sean Smith, mitigating, said: "He made the decision to take ten tablets in to the prison with him to try to assist him.
"He had been prescribed the tablets three weeks before..
"He had been trying to get himself off heroin."
Bowler was given a 14-week sentence, to be added to the two-and-a-half year sentence he is currently serving.
Judge Andrew Stubbs said: "You secreted a Kinder Egg up your backside.
"I accept that they were for your own use.
"But the danger of smuggling drugs into prison is well known.
"You could have subjected yourself to violence or possible threats.
"Possession of drugs inside the prison estate is far more serious than outside.
"They have a corrosive influence inside prison."
Last summer the Prime Minister was caught on camera trying to get his head around how prisoners ‘plug’ banned substances to get them into prison.
Mr Johnson appeared baffled when one officer at HMP Leeds had the awkward task of explaining the process.
Footage showed Mr Johnson looking at the body scan x-ray in disbelief saying: "A Kinder Egg?… Is that inside the?"
Justice Secretary Robert Buckland, who was standing beside, intervened to confirm the assumption.
But Mr Johnson still seemed unable to get his head around the whole phenomena.
He went on to ask: ‘He’s ingested it?’, to which the officer replied: ‘He’s plugged it’.
Still unable to come to terms with the picture of the Kinder Egg he was being shown, he burst out saying: ‘He plugged it?!’
Mr Johnson was then shown a second image.
This time it was an offender who had used the same ‘plugging’ technique to smuggle tobacco.
A shocked Mr Johnson said: ‘You are joking.’
Mr Johnson was visiting the prison when, according to its latest inspection, it was one of the most overcrowded in the UK and where in 2017 the majority of inmates claimed obtaining drugs was easy.
The scanner is said to have reduced the amount of drugs smuggled and therefore violence.
After Bowler's sentencing hearing, Detective Sergeant Lee Stowe, of Leeds District Prison Crime Team, said: “The supply and consumption of Illegal drugs in prisons fuels violence, exploitation and self-harm and creates significant risks to inmates and to staff.
“We will continue to work closely in partnership with the prison authorities to deter and disrupt the smuggling of drugs into prisons and conduct comprehensive investigations to ensure those, such as Bowler, who commit these offences are punished accordingly.”