Leading anti-racism groups have warned that the leader of the extreme-right Jobbik Party will face "heavy" protests should Theresa May let him enter the UK.
The Home Secretary is under pressure to ban Jobbik leader Gábor Vona ahead of his trip on Sunday, which coincides with the eve of Holocaust Memorial Day.
Mr Vona is expected to come to London for a planned forum for Hungarian expatriates in Britain.
Unite Against Fascism called for a counter protest to be held if the meeting goes ahead.
Sabby Dhalu, joint national secretary of Unite Against Fascism, said groups like Jobbik had "no place" in the UK and called on "all democrats to reject" the group this weekend.
An activist told the Guardian that protestors were likely to take to the streets and attempt to block Mr Vona's path to the venue in central London if he is allowed in.
On Thursday Hope Not Hate petitioned home secretary Theresa May demanding she deny Vona entry to the country.
The petition was signed by more than 14,000 people, and handed into the Home Office.
A joint letter from the Board of Deputies, Community Security Trust, London Jewish Forum and Jewish Leadership Council was sent earlier in the week. It stated that Jobbik has a “history of committing violent acts against minorities such as the Roma community and frequently espouses virulent antisemitism” and called for Mr Vona's address to be banned.
Politicians have also condemned Vona's trip to the UK.
Claude Moraes, Labour MEP for London, said: "I've seen Jobbik up close they are racist, antisemitic, anti-Roma fascists.
“We don't want them spreading their poison in our great international city London."
Jobbik have 43 of the 386 Hungarian parliament's seats, and three in the EU parliament.
London Assembly member Andrew Dismore has written to Mrs May demanding that Vona is banned from holding the event.
He called Jobbik "the most powerful outwardly fascist political party in Europe.”