Gabor Vona, leader of the Jobbik party, is due to meet with Hungarian citizens in London today to "discuss the upcoming Hungarian elections".
Activists, who have collected 13,000 signatures on a petition, have called on Home Secretary Theresa May to ban Vona from entering the country.
The Home Office have refused to intervene. British Labour MEP Claude Moraes said: "As a London MEP I've seen Jobbik up close. They [the Jobbik party] are racist, anti-Semitic, anti-Roma fascists - we don't want them spreading their poison in our great international city, London."
The Jobbik party is accused of promoting extreme, anti-Semitic views, and fuelling hatred against Jewish and Roma communities.
Vona's visit comes on the eve of Holocaust Memorial Day.
Nick Griffin, the BNP leader, said last month that his party and Jobbik shared a "common core set of values" and he was seeking a formal alliance.