The BBC Trust rejected appeals against their decision to allow BNP leader, Nick Griffin, to appear on Question Time, following an emergency meeting last night.
They decided that it was "a question of editorial judgment".
Mark Thompson, director general of the corporation, has defended the decision, saying that only governments could decide which organisations should be banned from the airwaves.
Writing in the Guardian today, he challenges the government to impose a broadcasting ban if they want the BBC to go against its "central principle of impartiality", referring to Margaret Thatcher's ban on Sinn Fein in the 1980s.
He claims that Griffin was invited to appear not from "some misguided desire to be controversial" but because of the public's right "to hear the full range of political perspectives".
Thompson says: "The case against inviting the BNP to appear on Question Time is a case for censorship."
Peter Hain, Welsh secretary and anti-apartheid campaigner, who wrote to the BBC questioning the legality of their decision, contested Thompson's argument, saying: "He is dodging the fact the BNP is a racist, fascist party in complete contradiction to the BBC's own equal opportunities and anti-racist policies. The BBC are in total denial about their gifting of a massive early Christmas present to the BNP. This is probably the worst decision the BBC has made in recent times."
Griffin - an MEP - arrived in London from Strasbourg and will prepare for the show at a secret location. Griffin said he admired Thompson's "personal courage" for inviting him, and said that it was his chance to "take on the corrupt, treacherous swine destroying our beautiful island nation".
Protesters are expected to picket BBC Television Centre in London all day ahead of the appearance.
