The decision to invite Nick Griffin, the British National party leader, on to tomorrow's edition of Question Time is obviously an editorial judgment – and one for which the BBC and I will certainly be called to account. But it is not a decision like the running order for this morning's Today programme or the line-up of stars on this season's Strictly Come Dancing.

Those who argue that, while the BBC is right to feature BNP politicians occasionally on the air, it is nonetheless wrong to have invited the party's leader on to Question Time, fail to understand not just the programme itself, but the reality of what the BBC's central principle of political impartiality means in practice.

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 – from the Scottish Socialists and Respect to the Green party – also take their seats on the stage. Question Time is the most prominent programme of its kind on British television, and we carefully study the support gained in elections by each of the parties, large and small, before deciding who to invite and how frequently they should appear.

It is a straightforward matter of fact that, with some 6% of the vote and the election of two MEPs in this spring's European elections – and with some success in local elections as well – the BNP has demonstrated a level of support that would normally lead to an occasional invitation to join the panel on Question Time. It is for that reason – not for some misguided desire to be controversial, but for that reason alone – that the invitation has been extended.

For the BBC to say to the BNP (or indeed to any political party), "Yes, you've met the objective criteria for appearing on Question Time, but we have decided that in your case it would be more appropriate if you didn't, but instead appeared on Newsnight or Panorama," would be for us to deny them parity with other parties, presumably on the basis of our own, or somebody else's, qualitative political judgment about the BNP.

That isn't impartiality, it is its opposite. It would be contrary to our obligations under the BBC's charter; and contrary, I believe, to the British public's expectations of us. It would be wrong.

Does that mean that we believe the BNP should not be challenged? Of course not. They should be challenged as tenaciously and as searchingly as any other political party – and I believe they are when they appear on the BBC. From news coverage to hard-hitting, and indeed award-winning, investigative journalism, we have probed both the BNP's stated policies and some of the views of the party's leaders and supporters that are expressed only behind closed doors.

But Question Time is the public's chance to challenge the politicians – that is why it is so important that they should sometimes be able to hear and interrogate politicians from the relative fringes as well as from the mainstream.

Political parties, of course, have the right to be treated fairly and evenhandedly by the BBC. But the central right we are upholding in this decision is the public's right to hear the full range of political perspectives, to hear other members of the public putting those perspectives to the test, and then to form their own conclusions. Excluding any party with demonstrable popular support from taking part in the programme would be to curtail this public right.

The case against inviting the BNP to appear on Question Time is a case for censorship: the case, in other words, that (in the opinion of those who make it) the BNP's policies are so abhorrent and so liable to sow hatred and division that they should be excluded from this form of public discourse altogether.

Democratic societies sometimes do decide that some parties and organisations are beyond the pale. As a result, they proscribe them and/or ban them from the airwaves. The UK government took exactly this step with specific parties and organisations in Northern Ireland in the 1980s.

Many would argue that proscription and censorship can be counter-productive, and that it is usually better to engage and challenge extreme views than to try to eliminate them through suppression. My point is simply that the drastic steps of proscription and censorship can only be taken by government and parliament. Though we argued against it, the BBC abided by the Northern Ireland broadcasting ban in the 1980s, and, if the BNP were proscribed, the BBC would abide by that decision too, and the BNP would not appear on Question Time.

But that hasn't happened, and until such time as it does it is unreasonable and inconsistent to take the position that a party like the BNP is acceptable enough for the public to vote for, but not acceptable enough to appear on democratic platforms such as Question Time. 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.

At the heart of public service broadcasting is the idea of public space – of programmes and services available to all, and within which people can encounter not just ideas and attitudes that accord with their own, but ones that are utterly different from theirs, and with which they may profoundly disagree. As the present debate about Question Time demonstrates, maintaining this space is sometimes difficult and controversial. It is also essential, if we really want the public to engage in the democratic debate about the great issues of the day.