Geert Wilders, the leader of the right-wing Freedom Party in the Netherlands, is not so much an unlikely as an incredible standard-bearer for liberty. His most prominent political stance is his opposition to what he terms “Islamic invasion”. He is the authentic voice of provincial populism and even xenophobia. He encapsulates the adage of Harold Macmillan that criticism in politics is never inhibited by ignorance.

Mr Wilders' remorseless themes are opposition to immigration and condemnation of Islam. He depicts Islam as monolithic, monocausal and monomaniacal. Its founder is a “terrorist” and a “war criminal”. Mr Wilders compares the Koran to Mein Kampf for its incendiary content, and demands that it be proscribed. With an irony so clumsy that it can be lost only on himself, he declares 2009 “a year to defend free speech”.

He is a very unlikely ally of liberal values. Indeed, his notoriety in the Netherlands, and the adulation that he enjoys among nativist movements elsewhere, is based precisely on his antipathy to basic liberal precepts. His opposition to Islam is a demand for cultural uniformity rather than a defence of secularism. His talk of the universality of human rights is a cover for restricting their reach.

Instead of calling for a common citizenship that makes no distinction on the grounds of race or creed, Mr Wilders demands that the United States and European nations “stop appeasing Islam and start fighting together against the rapidly increasing Islamisation of Europe”.

There is almost nothing on which Mr Wilders cannot be relied upon to be inflammatory, ill advised and illiberal, and this is precisely why the Home Secretary is wrong to deny him entry. Mr Wilders is an elected politician in a member state of the European Union. Freedom of speech, association and travel is part of the political culture of Europe.

For all the obvious hollowness of Mr Wilders' credentials as a defender of free speech, the cause is a good one. It is a common notion that the right to free speech must be held in balance with the requirement to avoid needless offence. That is a mistake. The right to oppose, mock, deride and even insult people's beliefs is essential to a society where bad ideas are superseded by better ones. There is no right to have one's emotional sensibilities protected, for it is no business of government to legislate for people's feelings. Mr Wilders' views are obnoxious, and (not but) his freedom to express them must be defended. It is regrettable that Mr Wilders faces not just ostracism but prosecution in the Netherlands because of his comments about Islam.

The Home Office judges that Mr Wilders' presence in the UK would threaten public order and has banned him from entering the country. Last year Sheikh Yusuf al-Qaradawi, a Muslim cleric with inflammatory views on Jews and homosexuals, was denied a visa to visit the UK. Mr Wilders' politics are no less inflammatory.

But that is not enough to warrant a ban. Demagogic speech is a test of the liberal political rights on which the culture of a liberal democracy rests. Let Mr Wilders exploit them. His political posturing is so self-evidently preposterous that, if he is permitted to speak freely, he will be arraigned before the best court in the land - the court of public opinion.