Now that this week's edition of Question Time or the "Nick Griffin Road Show" (as it might more appropriately be called) is over, white metropolitan liberals are proclaiming it a triumph for free speech. But in truth it was nothing of the kind.

For one thing, there was no issue of free speech involved. Nick Griffin is entitled to make speeches, write pamphlets (he did one a few years ago about how the media is run by a Jewish cabal), author books, go on television, be interviewed on the radio and have a website.

Many of us are opposed to his views and do not want them to spread, but we do not dispute his right to express them just so long as he stays within the law. But he has no God-given constitutional right to appear on any particular television programme. That is a matter of judgement. And in choosing to put him on Question Time the BBC exercised the wrong judgement.

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The argument, peddled by Mark Thompson, Director-General of the BBC, that Griffin had to go on because his vote went up in last European elections, is just false. The BNP vote actually went down in these elections. It gained two seats, not because of any great upsurge in support, but because everybody else's vote collapsed and the elections were fought under PR. The BNP's relatively increased proportion of the vote got it two seats, not an increase in support.

Nick Griffin may be complaining now that the show was biased against him. But in fact, the programme was a triumph for the BNP. Never before has it had wall-to-wall publicity on all the media for so many days. If it had been obliged to pay for that kind of media exposure, it would have cost it a fortune. As it is, Nick Griffin and the BNP have seeped into the consciousness of the most apolitical Britons.

A friend described picking up her children from school that day and hearing ordinary mums with no interest in politics who would not normally watch Question Time intoning confidently that this was "BNP Day" on Question Time.

With that level of exposure, there comes a level of legitimacy. It does not matter that Griffin gave a poor performance. No matter that London-based middle-class journalists competed to denounce him in the newspapers the morning after. What people will remember was that last Thursday was indeed "BNP day" and the party and its views were somehow accepted into the mainstream.

More than 22 years ago, I appeared on Question Time as a young parliamentary candidate. For days afterwards people, black and white, came up and congratulated me. It did not matter what I said. My very appearance on the programme signalled acceptance by the political mainstream in a way that appearing on no other political programme could have achieved. That was why it was wrong to have Nick Griffin on the show.

Furthermore, while metropolitan press coverage saw the show as a disaster for Griffin, it is clear from the comments on blogs and phone-ins that many potential BNP supporters saw Griffin as being ganged up on and treated unfairly by metropolitan elitists. It may seem ludicrous to many of us for Griffin to be presenting himself as victim. But it will resonate with many whites outside of the metropolis who see themselves as victimised and marginalised by economic uncertainty and new, frightening, multicultural Britain.

Furthermore, it is noticeable that at least half the show was taken up with discussing immigration. And even though the other politicians were aggressive about denouncing Nick Griffin, they were equally aggressive in declaring that there was indeed an immigration "problem" that only they knew how to solve.

The Tory spokeswoman Baroness Warsi called for a cap on numbers (impossible without abrogating the United Nations Convention on Human Rights and withdrawing from the European Union). The Lib Dem spokesman Chris Huhne complained about the numbers of Eastern Europeans who had come here (odd from a self-confessed Europhile) and my colleague, Jack Straw, was reduced to bleating about how tough New Labour had already been.

So although they were denouncing the man, the debate stayed firmly on territory delineated by the BNP. Thus does the introduction of the BNP into mainstream discourse drag our politics further rightward.

The appearance of Nick Griffin on Question Time may have brought it record viewing figures. But at what cost? It is not just the thousands of pounds of police time or even the tens of thousands of pounds of free publicity for a fascist party.

We are moving through a recession, which will undoubtedly get worse (in terms of its impact on ordinary people's jobs and services) before it get better. These are the classic conditions for the rise of a fascist party. In retrospect, Nick Griffin's appearance on Question Time may well be seen as significant. But its significance may not be a happy one.

Diane Abbott, Labour MP for Hackney North and Stoke Newington, was the first black woman elected to the British Parliament