He appeared alongside French National Front MEP Bruno Gollnisch and Italian far-Right MEP Roberto Fiore to discuss policies ahead of June's European Parliament elections.

Mr Gollnisch was convicted in 2007 of Holocaust denial after he questioned the existence of the gas chambers.

Mr Fiore, leader of the Forza Nuova party, was hunted by police after the Bologna station massacre in which 85 people died in 1980. Far-Right terrorists were blamed for the attack.

He was later cleared of involvement but convicted of "subversive association" and sentenced to nine years in prison.

Mr Darby, aide to BNP London Assembly member Richard Barnbrook, today insisted there was nothing wrong with him meeting the pair.

"Nelson Mandela was convicted of far worse crimes than them," he told the Evening Standard. "He was a convicted terrorist, as were Sinn Fein, who have been invited into government. So that won't wash."

When put to him that members of the conference were seen making Nazi salutes, Mr Darby replied: "I can't do anything about that. There were 500 people and three of them were caught doing that stupid behaviour."

He added: "Every time European nationalists meet up we are accused of being Nazis. I only work part-time. What I do in my spare time is up to me. I have done nothing wrong. I don't have a conviction. What have I done? Met two members of the European Parliament. Labour politicians see them a lot in the debating chamber."

When asked if he felt it was acceptable for an MEP to have denied the Holocaust, he replied: "I don't think it's right, but why don't you ask that question to these Labour and Tory MEPs who sit in the same chamber?"

Labour's leader on the London Assembly, Len Duvall, said: "People who associate with those who promote Nazi ideology and who offer simplistic solutions to some of Britain's most complex problems have no place in modern political life."

A spokesman for anti-fascist organisation Searchlight, which took the photos, said: "Darby's wages are paid by London taxpayers, and yet here he is swanning around Milan, glad-handing convicts and Holocaust deniers."