The former leader of Britain First has been jailed for eight weeks for breaching a court order not to enter a mosque or encourage others to do so.
Paul Golding, 34, who recently stepped down as head of the group for family reasons, admitted contempt of court.
London's High Court heard that nine days after the injunction - which prohibited entering any mosque in England and Wales without prior invitation - was imposed in August this year, Golding drove four Britain First members to the Al-Manar Centre in Cardiff for a "mosque invasion".
Golding stayed outside and there was no violence but there was a verbal confrontation between his four colleagues and a mosque trustee.
James Weston, counsel for the Chief Constable of Bedfordshire Police who brought the committal application, said that members of the mosque found the conduct provocative and unnerving and were concerned that it could have escalated if prayers had not been over.
Sentencing Golding on Thursday, Judge Moloney said the breach was a "deliberate and cynical defiance" of the court's order as well as an affront to the Muslim community not merely in Cardiff but throughout the country where Britain First might circulate its propaganda.
Golding will serve four weeks of the sentence.
The judge said Golding, who was leader at the time, played an active part in the incident, taking photos as well as driving the others.
"There can be no doubt that he thereby broke the injunction by instructing or encouraging those men to enter the mosque."
He said it was not true that Golding, who made an unreserved apology, did not understand the terms of the order - and his assertion that there would be no repetition would have more force it he had not made a similar promise in 2015.
He added: "Such an injunction is granted to prevent serious anti-social behaviour. This particular injunction was granted not merely to protect certain individuals but to preserve public order in the widest sense and throughout the country.
"The conduct restrained was by its nature of an extreme kind, calculated to increase tensions between different members of the community of this country, particularly to affront the Muslim community in relation to their religion.
"Such conduct was plainly calculated to give rise to the risk of provocation and violence and further extremism and tension on all sides of the community.
"These are most serious matters at the present time."