In a year's time, you will arrive in the blank neon tedium of Heathrow and approach the passport controls with the usual weary anxiety. Only now, something will be different. If you are white or Chinese or black, you will be filtered into a fast-streaming, security-loose queue. If you are Asian, you will join the high-security line, to be prodded and poked for terror signs. Your offence? Flying While Asian.

That was the government vision last week - now, it seems, probably watered down under pressure from our EU partners. (With John Reid in charge, you can't be too sure). But it seems the crime of FWA is now being policed privately instead: this weekend, a planeful of panicked British tourists refused to let their flight take off until a pair of totally innocent brown-skinned men speaking "a foreign language - maybe Arabic" were carted away by the police.

Last week, I discussed the spike in anti-Semitism in Britain. Today, let's look at its ugly twin - the rise in suspicion and hatred of Muslims. In the shadow of a burning World Trade Center, anti-racist liberals are constantly butting our heads against a difficult question. How do we express our hatred of the Talibanist jihadi death cult, without contributing to hatred of ordinary democratic Muslims like those passengers who loathe these fundamentalists just as much as the rest of us? Where is the line between hatred of woman-and-gay-hating Islamism and hatred of the democratic followers of Islam?

It's not just those panicked passengers who have crossed this line. There are intellectuals on the British right who are propagating a conspiracy theory about Muslims that teeters very close to being a 21st century Protocols of the Elders of Mecca. Meet Bat Ye'or, a "scholar" who argues that Europe is on the brink of being transformed into a conquered continent called "Eurabia".

In this new land, Christians and Jews will be reduced by the new Muslim majority to the status of "dhimmis" - second-class citizens forced to "walk in the gutter". This will not happen by accident. It is part of a deliberate and "occult" plan, concocted between the Arab League and leading European politicians like Jacques Chirac and Mary Robinson, who secretly love Islam and are deliberately flooding the continent with Muslim immigrants. As Orianna Fallacci - one of the best-selling writers in Italy - has summarised the thesis in her hymns of praise to Ye'or, "Muslims have been told to come here and breed like rats."

Rather than dismissing her preposterous assertions, high-profile writers like Melanie Phillips, Daniel Pipes and Niall Ferguson laud Ye'or as a suppressed hero, silenced by (you guessed it) "political correctness". Her name is brandished as a gold standard in right-wing Tory circles. It's interesting that writers so alert to anti-Semitism have lent their names to an ideology that is so startlingly similar. In this theory, the Star of David has simply been replaced by the Islamic crescent. If the term has any meaning, this is authentic Islamophobia, treating virtually all Muslims as verminous sharia-carriers. So why are these people still treated as serious and sane by the BBC and its editors?

But criticising these people as Islamophobes has become difficult, because at the other end of the political spectrum there is a hard core of supposedly left-wing groups who have abused the concept. They smear every rational critic of Islamic fundamentalism as racist. Look, for example, at the popular website Islamophobia Watch. According to them, if you are find Iqbal Sacranie - a religious leader who says gays spread disease and that death is "too good" for Salman Rushdie - disgusting, you are an "Islamophobic bigot" on a par with Ye'or. They obsessively snipe at left-wing atheists like Peter Tatchell, even calling him "pro-Nazi" because he uses "the term 'Islamism' without distinguishing between its reformist and violent wings". Yes, it's true - Tatchell fails to draw a distinction between the people who will lash and stone gays after winning at the ballot box and the people who will lash and stone gays after seizing power in a coup. What a Nazi.

In a mirror image of Bat Ye'or, IslamophobiaWatch - and the dense chunk of the left who adopt their mentality - unwittingly treat Muslims as a homogeneous bloc represented by their most reactionary elements. To them, an attack on one Muslim is an attack on all. If you condemn Sheikh al-Qaradawi - an Islamic cleric who endorses wife-beating - then you are simultaneously condemning a Muslim woman who is being beaten by her husband. They cannot see that in reality you are defending her.

It's only by looking at these two extremes that the way to criticise Islamism without encouraging bigotry becomes clear. The central rule is: never act as though all Muslims are essentially the same. Like all religions, Islam is based on a dense, contradictory and often ludicrous Holy Book that is being constantly reinterpreted. Some Muslims will follow Mohamed's calls to kill Jews and infidels and ignore the passages where he preaches peace; some Muslims will do exactly the opposite. Superstition is elastic, and it can be stretched in almost any direction.

Like Muslims everywhere, the British Muslim community is polarised between these interpretations. The recent polling shows that some 54 per cent believe the Government is not doing enough to tackle Islamic fundamentalism - more than the rest of us. That's because these moderate Muslims (especially women) see the fanatical fringe - like the terrifying 6 per cent who believe the 7/7 massacres were justified, or the 34 percent who believe British Jews are "a legitimate target" - more clearly than everyone else. As an atheist, I'm not going to get into the game of saying one side is offering the "true" Islam. I think they are both false, but I know which one I want to prevail and I know there is a 1,000 years between Osama bin Laden and (say) Irshad Manji, the lesbian Muslim liberal.

The government's proposal to separate Asians from other passengers at airport terminals - or our freelance attempts to do so - breach this basic rule. It refuses to distinguish between a jihadi who dreams of blowing up the Ministry of Sound because it is full of "slags" - a recent case before the Old Bailey - and a Muslim who parties there every Saturday. Every refusal to draw a distinction places a weapon in the jihadis' hands. Their siren argument to young British Muslims is: you will never be one of them. See how they hate and humiliate you?

So while it might seem absurd at first glance to screen an elderly Chinese woman for suicide-bombs with the same vigour as a young Asian man, it actually makes us safer in the long term. It holds back hundreds of borderline young Muslim men from tipping over into alienation and sympathy for jihadism. This is the rule for tackling real Islamophobia: remember that liberal Muslims the first victims of jihadists and our closest allies in the fight against them. Only a fool would begin to harass and harangue them now.

j.hari@independent.co.uk