What Rowan Williams wishes upon us is an abomination and I write here as a modern Muslim woman. He lectures the nation on the benefits of sharia law – made by bearded men, for men – and wants the alternative legal system to be accommodated within our democracy in the spirit of inclusion and cohesion.

Pray tell me sir, how do separate and impenetrable courts and schools and extreme female segregation promote commonalities and deep bonds between citizens of these small isles?

What he did on Thursday was to convince other Britons, white, black and brown, that Muslims want not equality but exceptionalism and their own domains. Enlightened British Muslims quail. Friends like this churchman do us more harm than our many enemies. He passes round what he believes to be the benign libation of tolerance. It is laced with arsenic.

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He would not want his own girls and women, I am sure, to "choose" to be governed by these laws he breezily endorses. And he is naive to the point of folly if he imagines it is possible to pick and choose the bits that are relatively nice to the girls or ones that seem to dictate honourable financial transactions.

Look around the Islamic world where sharia rules and, in every single country, these ordinances reduce our human value to less than half that is accorded a male; homosexuals are imprisoned or killed, children have no free voice or autonomy, authoritarianism rules and infantilises populations.

What's more, different Muslim nations claim to have their own allegedly god-given sharia. In Saudi Arabia, women cannot drive (What in Allah's name could the Koran have warned about cars?). In Bangladesh and Pakistan, they have no such bar to driving, although increasingly Saudi Wahabi Islam is taking over and we see Saudi sharia taking hold.

It is growing in influence here, too. Ten years ago, the only fully shrouded Muslim women around were from the Arab fiefdoms, the many wives of sheikhs often drawn by cartoonists to convey the absurdity and inhumanity of such cloaks. Now all of Europe has these girls and women rendering themselves invisible in public spaces. It is their elected sharia, so they claim without credibility. There is no agreed body of sharia, it is all drafted by males and the most cruel is now claiming absolute authority.

In Pakistan, on the statutes are strictures on adultery introduced by the military dictator Zia ul-Haq. Women activists in that country have given their lives protesting against the injustice of those laws where women suspected of adultery, or rape victims, are punished in hideous ways and the man goes free.

The Iranian theocracy changes its regulations from year to year, capriciously playing with the lives of females. The morality police hound women and girls, beat them up, imprison them for showing an ankle, walking too provocatively or singing in the streets. They fight back but are ground down eventually.

Two Iranian friends chose to die rather than live under the demeaning religious orders. Go to Afghanistan if you fancy a 12-year-old bride – a practice approved by the mullahs. That's sharia for you. Many women, gay men and dissidents came to Britain to escape Islamic tyrants and their laws. Dr Williams supports those laws and, by default, makes the refugees victims again.

Four years ago, a Saudi woman in her fifties came to my home. She was divorced from a Saudi prince who had sent her away and kept her children. What she said about sharia cannot be repeated. She had money, this princess, but no parental rights and she howled like a child in excruciating pain in my living room.

Yet, family disputes, says Dr Williams, would be easier, within sharia. For whom exactly? The polygamous men who live in this country, yes, certainly. Not for their wives who will be told that God intends them to lower their eyes and accept unjust verdicts.

Many will be sent back to bastard husbands or flinty-eyed mullahs will take their children away. In Bradford and Halifax, they may be forbidden to drive or work where men are employed. Adultery will be punished. I don't think we will have public stonings but violence of some sort will be meted out (it already is) with lawmakers' backing.

Sensing the drift in their direction, British sharia "experts" today shamelessly direct female medical students not to wash their forearms, essential to prevent the spread of infections, because that exposes their flesh.

Does the Archbishop even know that sharia comes in many guises and that several schools of jurisprudence have their own versions? The list is long – Hanafi, Maliki, Shafi, Hanbali, Jafari, Salafi and on and on. Ayatollah Khomeini preferred his DIY set of crimes and punishments when he came to power.

No women are allowed to be imams or serious jurists, so cannot help make their own fair and free set of female-friendly sharia. All the systems insist on ultimate truths, hard certainties. Sharia cannot provide solutions to the complex challenges of modern life and many violate fundamental human rights as established by the United Nations.

Taj Hargey, a historian and Islamic theologian, runs the Muslim Education Centre in Oxford. He, with me, is a trustee of British Muslims For Secular Democracy which is attempting to educate Muslims out of authorised obscurantism and non-Muslims into a better understanding of the progressive and evolutionary nature of the practice of Islam.

He is incandescent that Dr Williams backs a perilous Islamic conservatism, already too powerful in Britain: "Sharia is nothing but a human concoction of medieval religious opinion, largely archaic and outmoded and irrelevant to life today. Most sharia contradicts the letter and spirit of the Koran, distorts the transcendental text."

During his sermons Dr Hargey explains to congregations that, for example there is no blasphemy in the Koran, that the Prophet himself allowed a man to mock the divine revelations. Apostasy, says the holy text, will be dealt with by Allah in the afterlife. Sharia policemen insist apostates should be tortured and killed.

Dr Williams says Muslims want the choice to opt for sharia. What he believes to be choice is, in truth, inner compulsion, the result of brainwashing which begins in the madrassas when girls and boys are young enough to mould.

I have often admired the Archbishop's lofty thoughts, his intellectualism, the passion for human rights, his guts when the Government needs to be chastised. But this time his kind indulgences betray his own invaluable principles and deliver Muslim women, girls and dissidents into the hands of religious persecutors – an unforgivable intervention, which I hope he now sincerely regrets.