The German leader of the socialist group in the European parliament has told of his personal anguish at a link made by the Conservative party’s most senior MEP between his party’s political philosophy and that of Adolf Hitler and the Nazis.

Udo Bullmann accused Briton Syed Kamall of tarnishing the memory of Social Democrats who fought national socialism, often at the cost of their lives.

“What Kamall said was outrageous, it is outrageous to the memory of the Social Democrats who fought the Nazis and died for it”, said Bullmann, who leads the Progressive Alliance of Socialists and Democrats to which Labour’s 20 MEPs belong. “He later apologised for it, but only half-way, how he did not want to hurt our feelings. It is unacceptable. I am personally offended by the ignorance Kamall has shown. It is shameful”.

In an interview with the Guardian, Bullmann, a former academic from Giessen, north of Frankfurt, told of his family’s long history in the social democratic movement, a branch of which his great-grandfather, a gardener, founded in his home town 127 years ago.

Facebook Twitter Pinterest Udo Bullmann said modern-day Tories do not realise that Europe is the answer to ‘fascist ideology’. Photograph: Patrick Seeger/EPA

The Social Democratic party was banned by Hitler in 1933, after which many of its leaders were sent to concentration camps. One of its most revered leaders, Kurt Schumacher – who in 1932 described the national socialist ideology as “a continuous appeal to the inner swine in human beings” – spent 13 years in Hitler’s camps, including Dachau.

The clash between the two leading MEPs emerged during a debate in the European parliament on the outcome of a recent EU leaders summit in Brussels.

I really don’t understand what has happened to this great country, the United Kingdom, that was fighting the Nazis. Udo Bullmann, MEP

Bullmann had lamented the lack of progress towards a Brexit deal and made a call for a second referendum in the wake of the march of nearly 700,000 people over the weekend. He had further pledged to fight “right-wing nationalism and extremism” wherever it developed in Europe.

Kamall, a supporter of the Leave vote who leads the European Conservatives and Reformists in the parliament, responded by criticising Bullmann’s approach, and arguing that members of Hitler’s party were “socialists” and “leftwing”.

Pointing to Bullmann, Kamall added: “They want the same thing as you.”

After a storm of outrage, 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.”

Guy Verhofstadt, the leader of the Alliance of Liberals and Democrats for Europe party, called for an apology, saying Kamall’s comments were an insult to “all those social democrats who fought against Nazis”.

Bullmann told the Guardian he had found the moment personally hurtful but also deeply politically worrying.

“This is something that never ever happens in this house, nobody dares to put us in any connection with this extremist rightwing fascist who ruined the whole of Europe,” Bullmann said. “I have to fight this behaviour.”

“It was only the social democrats that rejected the enabling act [that gave Hitler his dictatorial powers in 1933] and then the Nazis waited for them outside the room to put them in a concentration camp,” he said.

“But the second thing on mind is that I really don’t understand what has happened to this great country, the United Kingdom, that was fighting the Nazis. It was this great old party, the Tories, who were fighting the Nazis. It was Churchill. And they don’t now know what the Nazis were or the history of the last century. That is why they do not realise that Europe is the answer to that, to that idiotic barbarism, and fascist ideology.”

He added: “I now more and more understand how the Tory party is unable to get a decent future organised for their country. They don’t know their history.”

• This article was amended on 25 October 2018. Udo Bullmann is from Giessen, not Glessen as an earlier version said.