Linton Weirich, 36, has been reportedly targeted while holding his child during a visit from his girlfriend at Swaleside Prison in Kent on December 18 last year.
The baby fractured their skull, but has since been discharged in hospital.
While Kent Police have not named the prisoner attacked, The Sun reports this is Weirich.
He was moved from HMP Wandsworth following the video circulating on social media of him having sex with former prison officer Linda De Sousa Abreu.
Abreu, 30, of Fulham has been sentenced to 15 months in prison on January 6 this year.
The charge says that De Sousa Abreu “wilfully and without reasonable excuse or justification misconducted yourself in a way which amounted to an abuse of the public’s trust in the office holder by engaging in a sexual act with a prisoner in a prison cell”.
It happened at Wandsworth between June 26 and 28.
She was arrested at Heathrow Airport and was due to catch a flight to Madrid, having notified the prison of her travel plans.
A Kent Police spokesperson said: “Kent Police is investigating an allegation of assault at HMP Swaleside, which happened on 18 December 2024.
“It is reported a prisoner was punched by another inmate in an area used for visitors.
“The victim sustained head injuries and fell onto a three-month-old baby he had been holding.
“The baby suffered a fractured skull but has since been discharged from hospital.
“The suspect has been identified and an investigation is ongoing.”
