Deputy director general Mark Byford has said it is not the BBC's role to censor the BNP as criticism mounts ahead of the party's Question Time appearance.
He said the BNP's Euro vote meant the BBC had to allow it on as part of its "responsibility of due impartiality".
Cabinet minister Peter Hain had asked the BBC to rethink its invitation to the whites-only political party.
Ex-London Mayor Ken Livingstone said the BBC would bear moral responsibility for any "spike" in racist attacks.
And the Equality and Human Rights Commission said the party's membership rules were currently illegal and it should not be regarded by the BBC "as equivalent to other political parties which abide by the law".
Mark Byford defends the BBC's decision to invite the BNP to appear on Question Time
Some protesters have already gathered outside BBC Television Centre in London ahead of the appearance of British National Party leader Nick Griffin, who is a Euro MP, on the hour-long flagship BBC political programme Question Time.
Mr Livingstone told BBC Radio 4's Today programme: "Unlike any other party, when Nick Griffin speaks, or when they get elected in an area, what we see is an increase in racial attacks.
"He comes on, says his bit, does his bit, but for the angry racist it's the trigger that turns into an attack. And we first saw this when Enoch Powell made his 'rivers of blood' speech, there was a huge surge of attacks on black conductors on our buses."
But Mr Byford told the same programme: "They should have the right to be heard, be challenged, and for the public who take part in Question Time and the viewers to make up their own minds about the views of the BNP. It's not for the BBC to censor and say they can't be on."
Public's 'opportunity'
Mr Hain's appeal to the BBC Trust to stop Mr Griffin appearing was rejected on Wednesday.
The trust said it was a "question of editorial judgement" whether it was appropriate for the BNP to appear.
 |   |
And BBC director general Mark Thompson, writing in the Guardian newspaper, said the case against having the BNP on Question Time was "a case for censorship".
He said only governments could decide which organisations should be banned from the airwaves.
The BBC Trust has asked Mr Thompson to ensure the pre-recorded programme is within BBC guidelines.
The decision to allow Mr Griffin, whose party won two seats in the European Parliament in elections in June, has prompted an outcry among anti-fascist protesters.
Mr Thompson argued that where organisations were deemed to be "beyond the pale" they were proscribed and/or banned from the airwaves by - and only by - governments.
Question Time "carefully" studied the support gained in elections by each of the parties before deciding whom to invite and how frequently, Mr Thompson wrote in the Guardian.
|  If there is a case for censorship, it should be debated and decided in Parliament. Political censorship cannot be outsourced to the BBC or anyone else  Mark Thompson BBC director general |
"Question Time is an opportunity for the British public to put questions to politicians of every ideological hue. Politicians from the UK's biggest parties appear most frequently, but from time to time representatives of parties with many fewer supporters... also take their seats on the stage," he said.
"It is for that reason - not for some misguided desire to be controversial, but for that reason alone - that the invitation has been extended."
Mr Hain, a prominent anti-apartheid activist in his youth, had argued that the decision should be re-examined following a court case about ethnic restrictions on the BNP's membership rules.
The party has agreed to amend its constitution after the Equalities and Human Rights Commission sought an injunction, claiming the BNP was breaking the Race Relations Act by restricting membership to "indigenous Caucasian" people.
'Wrong platform'
Mr Hain argued that as it had not yet decided to change its constitution it was an "unlawful body" that should not be treated the same as "any other democratically elected body".
But the BBC said the case did not "legally inhibit" them from allowing Mr Griffin on the programme.
Speaking later Mr Hain said he objected to the BNP "appearing just like any of the other parties".
| I'm not racist, I've got loads of coloured friends but when every second house is African, they're moving in, got two cars, bought houses, what can you say? Middle aged man, Dagenham |
"It gives them the legitimacy, the respectability they crave from the BBC and that is what's shameful in my view."
Labour MP Diane Abbott - the first black woman to be elected to Parliament - told the BBC's Breakfast programme that Question Time was the wrong platform for the BNP.
"If you are a black or Asian viewer tonight and you switch on the television and you see Nick Griffin on Question Time - it's not a programme that's going to scrutinise his views, it's not that sort of programme, it's politics as entertainment.
"The first time I went on Question Time was 22 years ago. People were really pleased - they didn't remember what I said but they saw a young black woman on Question Time and they thought 'Now black people are part of the mainstream'. That is the effect the BNP will get tonight, that's what they want from it, that's why they're so thrilled."
Prank calls
Downing Street said on Monday that the prime minister did not oppose the BBC decision to have the BNP on the programme, saying Gordon Brown believed it was important to expose what a party stood for.
| QUESTION TIME GUESTS Jack Straw, Labour Baroness Warsi, Conservative Chris Huhne, Lib Dem Nick Griffin, BNP Bonnie Greer, Playwright |
And Culture Secretary Ben Bradshaw told the BBC's Media Show on Tuesday that most of the Cabinet did not share Mr Hain's opposition to Mr Griffin's appearance.
BBC Trustee Richard Tait said: "We have decided it would be wrong for the Trust to intervene in a programme not yet broadcast - even one as plainly controversial as this.
"To do so would undermine the editorial independence of the BBC - something we are strongly committed to preserve."
The BBC has also been defended by the comedian Russell Brand, who resigned from Radio 2 after the row over prank phone calls to the actor Andrew Sachs.
| FROM THE TODAY PROGRAMME |
Writing in the Sun newspaper, Brand said his former employers were "right to grant a forum" to the BNP and that the audience should be allowed to draw its own conclusions.
BNP spokesman Simon Darby said the party would use its own security to get Mr Griffin safely inside the building and there would be no counter-demonstration by the BNP.
And Mr Griffin told The Times: "I thank the political class and their allies for being so stupid. The huge furore that the political class has created around it clearly gives us a whole new level of public recognition."
