Tommy Robinson has been jailed for 13 months for breaking contempt of court laws.
His sentence can be revealed for the first time after The Independent and other media outlets fought a reporting restriction put on the case at Leeds Crown Court.
Robinson, whose real name was listed on court documents as Stephen Yaxley-Lennon, was arrested outside the court on Friday.
He admitted committing contempt of court by publishing information that could prejudice an ongoing trial.
The 35-year-old was already subject to a suspended sentence for committing contempt during a rape trial in Canterbury last year, and had been told that if he fell foul of the law again he would go to prison.