Conservative MP Daniel Kawczynski is facing calls to be suspended for sharing a platform with some of Europe's leading far-right politicians.
Labour said he should lose the whip for speaking at the same event as Hungary's nationalist PM Viktor Orban and Italy's ex-deputy PM Matteo Salvini.
The party said it was "disgraceful" that he was associating with "anti-Semites, Islamophobes and homophobes".
But the Polish-born MP said the reaction to his trip was "hysterical".
He said Mr Orban, who has been prime minister of Hungary since 2010 and Mr Salvini, a former deputy prime minister of Italy, "represent serious ideas and concerns, some of which are shared by many citizens of the UK".
Both politicians are strongly Eurosceptic and hostile towards immigration, particularly from Muslim countries. Mr Orban has won three successive elections while Mr Salvini remains an influential figure in Italian politics despite his League Party quitting government last year.
Mr Kawczynski was among 22 speakers at the National Conservatism event, currently taking place in Rome.
Other participants include Marion Marechal, the niece of Front National president Marine Le Pen, and representatives of Poland's Law and Justice Party and Spain's Vox Party.
The MP for Shrewsbury and Atcham defended his participation, saying he would be making the case for nation states against European federalism.
"Orban and Salvini are not to everyone's tastes, of course," he said in article for The Spectator.
"And I don't agree with each and every one of their policies. But I am not Hungarian or Italian and both leaders have been elected on huge popular mandates in their countries. They represent serious ideas and concerns, some of which are shared by people in Britain."
Mr Kawczynski, whose attendance was first reported by the Guardian, added: "Perhaps those criticising the event should take a more inquisitive approach rather than simply attacking its existence and maligning elected politicians from other countries who are due to speak.
"If so, they might discover why the vast majority of Europeans feel more loyalty to their own countries than the abstract idea of a federal European super-state."
Labour's Andrew Gwynne said the prime minister must clarify whether the MP was given permission by Tory whips to go to the event.
"It's disgraceful that just one week after Holocaust Memorial Day Daniel Kawczynski has shared a platform with anti-Semites, Islamophobes and homophobes," he said.
"He should be immediately suspended from the Conservative Party."
Leading Jewish groups, such as the Board of British Deputies and the Jewish Labour Movement, has also called on him to be disciplined.
"If the Conservative Party fails to discipline Mr Kawczynski, it runs the serious risk of the public assuming that they share his views on association with such people," the Board of Deputies said in a statement.
Crossbench peer, and former Labour MP, Lord Mann, who is the government's anti-Semitism tsar, said the MP's actions were "unwise" and Mr Kawczynski should be laying down an "unequivocal challenge" to the views represented at the conference.
And fellow Conservative MP Andrew Percy said his colleague's behaviour was "wholly inappropriate", telling the BBC that he had advised him against attending the event. He said he would be raising the matter with party officials.
Correction 20th February 2020: An earlier version of this article described Matteo Salvini as Italy's leader and this has been amended to clarify that he is the former Deputy Prime Minister.