The credibility of David Cameron’s new alliance in the European parliament is cast into fresh doubt today as the Observer reveals damning new evidence about its Polish leader’s past.

The allegations, which threaten to do serious damage to the Tory leader, centre on Michal Kaminski, a rightwinger chosen this month to chair the new and supposedly mainstream European Conservatives and Reformists group, of which the 25 Tory MEPs are members.

Opponents of Kaminski, 37, claim he has shown homophobic and antisemitic tendencies at odds with Cameron’s vision of a new tolerant Tory party. In particular, they say Kaminski was active in efforts to block an apology by his countrymen in 2001 for the massacre of hundreds of Jews in Jedwabne in July 1941. He denies this.

Speaking to this paper Kaminski also insisted he had never given an interview to a far-right Polish journal, Nasza Polska, during which he allegedly said Poles should not apologise for the Jedwabne pogrom until the Jews said sorry for collaborating with the Soviets.

“I never did an interview,” Kaminski insisted, adding that he “never tried to stop” an apology. But investigations by the Observer call those denials into doubt. Residents of Jedwabne at the time – backed by Polish journalists who covered the story – say Kaminski is misrepresenting his past role.

Footage of a television news bulletin from 5 March 2001 shows Kaminski reacting to news that the then President Aleksander Kwasniewski was to issue an apology and saying: “I think that Mr President can apologise but for other things. He should withhold apologies for Jedwabne.” The editor in chief of Nasza Polska, Piotr Jakucki, confirmed that Kaminski gave the 2001 interview.

At that time Jedwabne was the focus of international press attention after an American professor, Jan T Gross, published a book, based on the accounts of local people, which concluded that Poles, with the help of some occupying Nazi troops, locked hundreds of Jews into a barn, and set it on fire. But many people in Jedwabne and other parts of Poland, including Kaminski, believed the whole of Poland was being unfairly blamed for an unproven crime.

Maria Kaczynska, then a journalist with Gazeta Wspolczesna, recalls Kaminski’s role. “I remember all of this very vividly. I had to be in Jedwabne to write about him. I saw him in Jedwabne. He had a big folder and he pulled out a file, a petition calling on locals not to participate in apologies to the Jews.”

Kaminski also flatly denies having been involved in attempts to set up a committee aimed at defending the people of Jedwabne. “I had no involvement with them,” he said. However, Stanislaw Michalowski, the town council head at the time, said: “He was trying to set up a committee of Jedwabne defence but he failed.” Rafal Pankowski, who edits Never Again, an anti-racist magazine, said it was “incredible and appalling that Kaminski can lead a group in the European parliament that pretends to be mainstream and tolerant”.

In a letter in today’s Observer Kaminski calls claims that he is antisemitic “distressing” and insists he has spent “a lifetime of work supporting Israel and the Jewish community in Poland”.

“I have made it clear that the actions of some Poles in the Jedwabne massacre were horrific and criminal. The Polish people were also shattered by the Nazis. While we should share in commemoration I do not believe we should make the whole Polish nation culpable for the criminal acts of a small minority.”

Glenys Kinnock, the Europe minister, said: “This is another example of David Cameron’s inexperience and his willingness to leave Britain isolated. In the global downturn, it is more vital than ever that Britain remains at the heart of Europe. He needs to learn that he will not serve Britain’s national interests by resorting to isolation and extremism.”

Tories in Europe

Why has Cameron formed a new EU group?

In 2005, when campaigning to become leader, he promised Eurosceptic MPs he would quit the federalist European People’s party (EPP).

What is the problem?

He struggled to make a new group and ended up with allies on Europe’s hard right.

Does it matter?

Yes. Angela Merkel and Nicolas Sarkozy are angry that Cameron has left the EPP. It strikes important deals before EU summits.