Stephen Lennon, the leader of the English Defence League, has been jailed for 10 months for using another person's passport to travel to the US.
Appearing at Southwark crown court, the 30-year-old, who is also known as Tommy Robinson, pleaded guilty to possession of a false identity document with improper indentation.
He was charged in October and remanded in custody after appearing at Westminster magistrates court.
Members of the EDL have campaigned to have him freed, claiming he had been falsely imprisoned and that his incarceration was a political move.
They held a protest outside Wandsworth Prison, vigils across the country and set up a fund to cover his legal costs.
"Tommy is being held on remand, accused of conspiring to cause a public nuisance and passport irregularities following his trip to New York earlier this year," the EDL said in November.
"After over a month in prison, it's no wonder EDL supporters are asking why someone who has dedicated himself to standing up against extremism is being victimised in this way."
In December, the far-right group said Lennon's human rights were being infringed in prison and that he had been falsely accused of recruiting prisoners.
He was moved a high-security prison, further angering the group. "Tommy has now been moved to Milton Keynes Woodhill, a category 'A' prison where he will now share his time with murderers and 'real' terrorists," it said.
"The inmates of this prison are serving life sentences for a plethora of different offences, no doubt Islamic terrorism being a major reason behind the numbers that dwell there."
Since Lennon's incarceration, Kevin Carroll, deputy leader of the EDL, has been running the group.
In November, he stood for the British Freedom Party in the 2012 Police and Crime Commissioner elections for Bedfordshire Police, but came fourth with 10 percent of the vote.