Tommy Robinson was punched in the face by a 70-year-old inmate just days after being thrown behind bars, according to reports.
The English Defence League founder, 36, was reportedly "decked" by the elderly lag for acting "like a celebrity" in the showers at HMP Belmarsh.
Robinson, real name Stephen Yaxley-Lennon, was jailed for nine months for contempt of court after putting an ongoing child grooming trial at risk of collapsing.
He pleaded with Donald Trump to offer him asylum in the US and claimed he would be killed in a British prison.
But the right-wing extremist's words fell on deaf ears and he was punched within 72 hours of arriving at Belmarsh, south-east London, 10 days ago.
A source told the Daily Star Sunday: "There was a bit of pushing then one prisoner, a 70-year-old, very hard pensioner, decked Robinson with a punch to the jaw.
“He didn’t have any idea who he had hit, but he wasn’t going to back down. Those are the rules in Belmarsh, you fight or back down.
“It was all over in a flash. One of the prison officers asked what was going on, but he said he slipped so no official report was made.
“If you start acting as though you are big time, someone will take you down.
"If you come into the prison and start acting like you are the boss they will take you down quickly.”
The Ministry of Justice said: “We do not comment on individuals.”
Robinson was jailed after filming defendants entering a child abuse trial at Leeds Crown Court and streaming the footage to millions of people on his Facebook profile.
This breached a reporting ban put in place to protect other child grooming trials involving 29 people.
He was jailed over the crime in May last year but freed on appeal, before being imprisoned again following a re-trial at the Old Bailey earlier this month.
