He swept to office on the back of a promise to break with tradition. But John Bercow, the Speaker of the Commons, has been accused of taking his shake-up of the role too far after he unleashed an attack on the British National Party.

Centuries of political impartiality demanded of the Speaker were rolled back in spectacular fashion by Mr Bercow yesterday, as he described the far-right party as "evil", adding that it was a "poison which we could well do without". It is thought to be the first time that a Speaker has ever launched such an assault on a legal party.

Mr Bercow, who was in the Speaker's chair to oversee the first ever UK Youth Parliament session taking place in the Commons, earned cheers and even a standing ovation from the delegates for his outburst. "I'm under absolutely no obligation whatsoever to be impartial as between the forces of democracy on the one hand and the forces of evil on the other," he said. "I do feel very, very, very strongly as someone from a Jewish background that the evil of the BNP is that its whole politics is based upon and driven by hate. That is a poison which we could well do without."

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While the Speaker is always expected to remain independent while overseeing business in the Commons, Mr Bercow may be saved from censure because Parliament was not technically sitting. The ornamental mace, which marks that the Commons is in formal session, was not in place during his comments.

Mr Bercow asked the delegates to reflect on history, during which political extremists had always sought to capitalise on discontent. "The Nazis in Germany in the 30s, and the neo-Nazis today, seek to scapegoat whole communities as they peddle their message of hate and scurrilously appeal for support," he said. "They depend on either ignorance, or apathy. And the counter to ignorance and apathy is education, interest and participation – all of which you have displayed today."

Simon Darby, the deputy leader of the BNP, said the outburst was "quite shocking" and broke the rules of the office of Speaker. "He is meant to be independent and this kind of outburst is quite extraordinary," he said. "We are hopeful of performing well at a general election and if he is still the Speaker, I do not see how he could possibly continue in the role. He has jeopardised his position."

Overseeing the Youth Parliament session, the first time non-MPs have ever been allowed to use the Commons, Mr Bercow said that MPs had been "vindicated" in allowing the chamber to be used by the 300 delegates, who were aged between 12 and 18. More than 100 were given the chance to speak in debates on tuition fees, crime, transport, the economy and the voting age. A small group of Tory backbenchers had tried to block the move.