When British Foreign Minister David Miliband was asked earlier this month to explain why Dutch MP Geert Wilders was barred from entering Britain in order to show his film 'Fitna', which criticises Islam, to a group of British MPs, he declared:

'We have a profound commitment to freedom of speech but there is no freedom to cry “fire” in a crowded theatre and there is no freedom to stir up hate, religious and racial hatred, according to the laws of the land.'

Milibrand's analogy is misplaced. If you apply it to other public figures, there are scores of current and former Danish MPs and public voices that would never be permitted to enter Great Britain. Such people have for years incited hatred against critics of immigration by comparing them with Nazis in Germany in the 1930s.

The analogy comes from a 1919 US Supreme Court ruling, but the argument behind the ruling differs from Miliband's application of it on a crucial point. What's more is that many would agree that the statement Oliver Wendel Holmes was trying to criminalise with his analogy is now completely legal. The case involved Charles Schenck, secretary of the US Socialist Party in Philadelphia. Schenck was being tried for circulating flyers that compared the US military draft during the First World War with slavery, and called on Americans to oppose the war using legal means.

Who today would prosecute an American who had protested against the recruitment of soldiers to fight in the wars in Iraq or Afghanistan? Holmes argued that Schenck's actions were made while the country was at war, and that they created a 'clear and present danger' that would undermine the US Army's efforts to win the war. Hundreds of Americans were taken to court. Holmes asserted:

'The most stringent protection of free speech would not protect a man in falsely shouting “fire” in a theatre and causing a panic.'

Drawing an analogy with socialist Schenck handing out flyers falls short in the Wilders case. What's more, Holmes later used an opposing argument to defend freedom of speech. But note that in defending the move to keep Wilders out, Miliband forgot that Holmes said you weren't allowed to yell 'fire' unless there actually was a fire. If there is a fire, or if there is smoke, then you have an obligation to draw everyone's attention to it.

Wilders's film is made up of documentary pictures, which makes it hard to reject them as false. What's more, the issue the film takes up – violence carried out in the name of religion – is a part of the European reality, which makes it a subject of heated discussion. You can argue that Wilders's depiction is one-sided and that it is propaganda, but Michael Moore does the same thing, and he wins film awards.

Harvard law professor Alan Dershowitz has pointed out that a more precise analogy for the flyers would have been if someone were standing in front of a theatre, handing out flyers stating that the theatre was unsafe, and urging people to stay away. But that analogy would have made it impossible for Holmes to defend the decision, and it would have made it difficult for Miliband to use the analogy as an argument for keeping Wilders out.

Despite the obvious logical flaws, the false analogy continues to be invoked each time someone wants to forbid unpopular points of view from being expressed. Doing so is foolish, because it tells us nothing intelligent about where the limits of free speech are.

If there's fire – or even just smoke – don't we have an obligation to make others aware of it?

Flemming Rose is a journalist and author. He was responsible for the 2005 publication of 12 cartoons depicting the prophet Mohammed that led to protests against Denmark.