The Conservative MEP sparked fury in the European Parliament chamber in Strasbourg as he responded to German MEP Udo Bullmann.
President of the Socialists and Democrats Group in the European Parliament, Mr Bullmann, claimed “right-wing nationalism” was “trying to undermine the European Union”.
Replying to the German politician, Mr Kamall said: “Mr Bullmann, I would remind you, when you talk about right-wing populists we have to remember that Nazi’s were national socialists.
“It is a strain of socialism. Let’s not pretend, it is a left-wing ideology, they want the same things as you, let’s be quite clear.”
European Parliament descended into chaos with MEPs repeatedly shouting over each other.
Former Ukip leader Nigel Farage could be heard in the background saying: “They won’t like the truth.”
Tory MEP snapped at the European MEPs adding: “You don’t like the truth do you?
“Come on, it is national socialism”.
One MEP shouted out the word “rubbish” before the Conservative MEP was branded an “idiot” by Dutch politician Frans Timmermans.
Speaking before the leader of ECR Group in European Parliament, Mr Bullmann claimed extremism was trying to undermine the European Union.
He said: “Right-wing nationalism, extremism is trying to undermine the European Union, is endangering the European Union.
“Fake news, we discussed this yesterday. The systematic form of undermining democracy has to be fought tooth and nail.
“The Austrian Presidency has missed a trick there, the Conservatives in this house will have to decide whether they are going to continue supporting democracy, and the European Union, or whether they are going to be the ones who are going to open the doors wide for the right-wing extremists.
“That is a decision ladies and gentlemen, that you will have to make. If you want to remaining reliable partners own the European Union, and we are going to hold you to that.”
The remarks from the Conservative MEP also sparked a furious debate between Mr Farage and the European Parliament President.
Mr Kamall’s comments were denounced by Guy Verhofstadt as “an insult to all the Social Democrats who fought against Nazism and died in concentration camps”.
Conservative MEP Mr Kamall apologised for his comments. He said: “I get tired of people saying Nazism is a right-wing ideology. I believe in freedom of speech, but if I have offended you – and clearly I have – I apologise unreservedly.”
Mr Kamall later tweeted: “It was not a personal comment aimed at any MEPs. I’ve apologised directly & unreservedly to Mr Bullman for any offence caused.
“I have upmost respect for anyone who stood up & fought against Nazism, Communism & any other kinds of extremism, regardless of political affiliation.”