Far-Right activists including members of Greece's Golden Dawn Party are planning a gathering in central London this weekend.
London Assembly member Andrew Dismore has written to Theresa May demanding that Gabor Vona, the leader of the right-wing party Jobbik, is banned from holding the event.
Mr Vona has denied claims by campaign group Hope Not Hate that he is meeting members of Greece's Golden Dawn party - who have openly expressed admiration for Adolf Hitler - and the British National Party (BNP) on Sunday.
His spokeswoman said that he is hosting a forum for Hungarian citizens ahead of an upcoming election.
She said that the claims were "false", adding: "He has no intention of meeting anyone from Golden Dawn or the British National Party.
"This is a forum for Hungarian citizens. This is not to do with anyone else.
"There are lots of Hungarians living in London and the election is coming up in Hungary."
The spokeswoman said that the meeting was happening in Holborn, central London, on Sunday afternoon.
Mr Dismore, who as a Labour MP founded Holocaust Memorial Day, told the Jewish News that Jobbik is "the most powerful outwardly fascist political party in Europe".
In his letter to the Home Secretary, he said: "I have no doubt that all decent thinking people in London will join with me in demanding that the Home Secretary says 'No to Jobbik' ".
"While I advocate the right to free speech in our society, this must also be balanced against other human rights.
"It is particularly important to note that his event is being held on the eve of Holocaust Memorial Day when the offence of his meeting will be all the more damaging."
Earlier this month BNP leader Nick Griffin told the Associated Press that his party was likely to forge an alliance with the two parties after the European Parliament elections in May.
Mr Griffin travelled to Athens to support Golden Dawn, which has seen a surge in popularity during the country's financial crisis, following accusations it is a criminal organisation.
The party denies any involvement in frequent street attacks against immigrants and other violence.
