Uefa is investigating claims the referee due to officiate the Champions League final next week was a keynote speaker at an event organised by a far-right politician. Szymon Marciniak, who also refereed the World Cup final, spoke at an event in Poland on Monday which was organised by Sławomir Mentzen. Mentzen, the leader of the Confederation party, is known for launching the political slogan “We stand against Jews, gays, abortion, taxation and the European Union”. In 2021 he made international headlines as the producer of a beer named White IPA Matters, which mocked the Black Lives Matter movement. Marciniak, 42, is due to referee the Uefa Champions League final between Manchester City and Internazionale on 10 June at Atatürk Olympic Stadium, Istanbul. But on 29 May he attended a gathering, under the title “Everest”, which was billed as a networking event complete with a beer drinking session. Anti-racist activists said the event was in reality an attempt to gain political support. Approached by the Guardian about Marciniak’s involvement, a Uefa statement said: “Uefa is aware of the allegations surrounding Szymon Marciniak and is seeking urgent clarification. “Uefa and the whole football community abhor the ‘values’ that are promoted by the group in question and takes these allegations very seriously. “A further announcement will be made tomorrow, after reviewing all the evidence.” His attendance at the event was first disclosed by the anti-racist organisation Never Again. Rafał Pankowski, the organisation’s co-founder, said: “We are shocked and appalled by Marciniak’s public association with Mentzen and his brand of toxic far-right politics. “It is incompatible with the basic values of fair play such as equality and respect. We call upon the referee to acknowledge his mistake. If he does not do it, we believe Uefa and Fifa should draw consequences.” Everest’s poster, distributed through social media, featured Mentzen himself drinking beer from a cup. Another poster featured Marciniak wearing a Fifa referee kit. A post by Mentzen announcing Marciniak’s presence at the event was shared on Marciniak’s Facebook fanpage. Marciniak was presented as a speaker on the event’s website alongside Mentzen, who subsequently praised Marciniak as a “genius speaker”. He is considered one of the most highly rated international referees of his generation. He refereed the 2022 FIFA World Cup final between Argentina and France in December in Qatar and the 2018 Uefa Super Cup between Real Madrid and Atlético Madrid. He was named “the world’s best referee” by the International Federation of Football History & Statistics in 2013. The Uefa inquiry into Marciniak’s involvement is expected to be undertaken by its chief of football, Zvonimir Boban, a former Croatian international, and its chief refereeing officer, Roberto Rosetti. The organisation, mindful of a slew of recent claims of racism in football, is planning to make another announcement on Friday. The Everest meeting in Katowice was advertised as a networking event for influential people to share their experience, with the most famous personality accepting the politician’s invitation being Marciniak, reports claimed. Mentzen, who also buys and markets beer, told one journalist that the event would be a different type of conference. “The atmosphere will be closer to a rock concert, a boxing gala than a lecture hall,” he was quoted saying. Mentzen, who spoke at the event, achieved notoriety in 2019 when he addressed a rally of the Confederation, a far-right group formed from extremist and nationalist parties and social movements. He told supporters: “We don’t want the Jews, the homosexuals, abortion, taxes and the European Union.” Mentzen’s brewery produced the beer White IPA Matters two years ago, accompanied by an advertisement showing a black barman drinking it in a bar decorated with Confederate flags, which sparked hundreds of complaints. Other beers produced by the firm include Polish Freedom and Keeper of Texas. In February, he became co-chair of the council of leaders of the Confederation party. Pankowski said Never Again was not calling for Marciniak’s removal from officiating the Champions League final, but hoped he would distance himself from the far right. Uefa has faced repeated criticism for failing to tackle racism. The former France and Arsenal forward Thierry Henry expressed his frustration in April at gestures by football’s authorities including Uefa such as No To Racism patches on shirt sleeves and joint team photographs before games. “I’ve had enough of looking at No To Racism on shirts. I’ve had enough of that lousy picture at the beginning of the game when we all come and focus on what’s on the field,” he said. Uefa was condemned last year after fining Sevilla £4,319 for fans’ racist abuse of the Manchester City player Rico Lewis.