Those who lament the decline of traditional Christianity often quote a line attributed to GK Chesterton (though there is no evidence of his authorship): "When a man ceases to believe in God he does not believe in nothing; he believes in anything." Like so many aphoristic cliches, it collapses after a moment's scrutiny – not least because nothing and anything are, in this context, effectively synonymous. A more telling version, formulated by my friend Nick Cohen, might be phrased thus: "If you can believe that wine and a wafer are the blood and body of Christ, you can believe anything." Even the no-nonsense Margaret Thatcher, we learn, was a devotee of mystical 'electric baths' and Ayurveda therapy.

But Thatcher was a mere dabbler compared with the present occupants of 10 Downing Street. Cherie Blair finds her devout Catholicism no impediment to flirtations with New Age spirituality, as she proved by inviting a Feng Shui expert to rearrange the furniture at No 10 and wearing a magic pendant known as the BioElectric Shield, which is filled with "a matrix of specially-cut quartz crystals" that surround the wearer with "a cocoon of energy" and ward off evil forces. Last December, amid the frenzy of media indignation at the discovery that the conman Peter Foster had helped the Blairs buy two flats in Bristol, some of us were far more alarmed by the disclosures about Foster's girlfriend Carole Caplin, who is employed by the prime minister's wife as a 'lifestyle guru'. Through Caplin, Mrs Blair has been introduced to an 86-year-old 'dowsing healer', Jack Temple, who treated Cherie's swollen ankles by swinging a crystal pendulum and giving her strawberry leaves grown within the 'electro-magnetic field' of a neolithic circle he has built in his back garden.

It was long assumed that Tony Blair, who wears his Christianity on his sleeve, did not share his wife's unorthodox enthusiasms. In 1999 he demanded the resignation of the England football coach Glenn Hoddle, who had told an interviewer that disabled people were paying off the bad karma they collected in previous incarnations. Blair thought this "offensive", though it was not discernibly more offensive than the doctrine of original sin held by many of his fellow Christians. Besides, from a Buddhist viewpoint Hoddle was quite correct: no less a figure than the Dalai Lama confirmed as much, but added that "if you live in a Christian country, you should keep these views to yourself. It is difficult to have a mish-mash of religions".

Not so, as Blair confirmed when he and his wife underwent a 'rebirthing experience' under the supervision of one Nancy Aguilar while holidaying on the Mexican Riviera in the summer of 2001. The prime ministerial mudbath was revealed by New Humanist contributor Tom Baldwin in a gobsmacking report for the Times:

"Ms Aguilar told the Blairs to bow and pray to the four winds as Mayan prayers were read out…Within the Temazcal, a type of Ancient Mayan steam bath, herb-infused water was thrown over heated lava rocks, to create a cleansing sweat and balance the Blairs' 'energy flow'."

Ms Aguilar chanted Mayan songs, told the Blairs to imagine that they could see animals in the steam and explained what such visions meant. They were told the Temazcal was like the womb and those participating in the ritual must confront their hopes and fears before 'rebirth' and venturing outside. The Blairs were offered watermelon and papaya, then told to smear what they did not eat over each other's bodies along with mud from the Mayan jungle outside.

The prime minister, on holiday just a month before the 11 September attacks, is understood to have made a wish for world peace.

Before leaving, the Blairs were told to scream out loud to signify the pain of rebirth. They then walked hand in hand down the beach to swim in the sea.

Mayan rebirthing rituals are not yet available in Britain through the National Health Service, but some of Cherie Blair's other weird obsessions have already been adopted as official policy. In January 1999 the government recruited a Feng Shui consultant, Renuka Wickmaratne, for advice on how to improve inner-city council estates. "Red and orange flowers would reduce crime," she concluded, "and introducing a water feature would reduce poverty. I was brought up with this ancient knowledge."

Two years later the government announced that, for the first time since the creation of the National Health Service, remedies such as acupuncture or Indian ayurvedic medicine could be granted the same status as conventional treatments. According to the Sunday Times, "the inclusion of Indian ayurvedic medicine, a preventative approach to healing using diet, yoga and meditation, is thought to have been influenced by Cherie Blair's interest in alternative therapy." An all too plausible suggestion, since Cherie is a client of the ayurvedic guru Bharti Vyas and officiated at the opening ceremony for Vyas's holistic therapy centre in London.

Although Tony Blair presents himself as a 'moderniser', many of his attitudes are actually a gruesome hybrid of pre-modernism and postmodernism – both of which have been particularly evident in his appeasement of fanatics who think that the book of Genesis is a factual account of humankind's origins. In March 2002 the Guardian revealed that Christian fundamentalists had taken control of Emmanuel College, a state-funded secondary school in Gateshead, and were striving to 'show the superiority' of creationist beliefs in their classes. One senior member of staff explained that "a Christian teacher of biology will not or should not regard the theory of evolution as axiomatic, but will oppose it." Blair is determined to build more 'faith-based schools', of course, and the news strengthened the misgivings of those who suspect that such academies proselytise rather than educate.

He had his chance to allay their fears in the House of Commons on 13 March last year, when Jenny Tonge MP asked if the prime minister was "happy to allow the teaching of creationism alongside Darwin's theory of evolution in state schools." A simple 'no' was surely the only possible answer; but it was not the answer he gave. Blair told Jenny Tonge that the creationists at Emmanuel College were doing a splendid job. "In the end," he said, "a more diverse school system will deliver better results for our children."

A few Labour backbenchers gawped in amazement as the significance of his remark sank in. Here was the leader of a supposedly secular, progressive government who, on being invited to assert that probable truth is preferable to palpable falsehood, pointedly refused to seize the opportunity – and indeed justified the teaching of bad science in the name of 'diversity'. He might just as well have trotted out the pernicious old maxim that ignorance is bliss, the last refuge of tyrants ever since God banished Adam and Eve from Eden for sampling the fruit of knowledge or the classical deities unleashed misery on the world through Pandora's box in revenge for Prometheus's heroic disobedience.

Perhaps Blair had meant what he said when he told his party conference in 1996 that New Labour took its inspiration from "the ancient prophets of the Old Testament"; more likely, however, he had been infected (even if unwittingly) by the cultural, moral and intellectual relativism of the postmodernists, and by the fashionable disease of 'non-judgmentalism'. As if to confirm the modishness of this affliction, his astonishing statement went largely unchallenged, even though it marked a new low in contemporary British political discourse. What if some schools informed their pupils that the moon was made of Swiss cheese? Would that be officially welcomed as another healthy consequence of Blair's 'more diverse school system'?

This is the enfeebling legacy of post-modernism: a paralysis of reason, a refusal to observe any qualitative difference between reasonable hypotheses and swirling hogwash. At a time when countless loopy creeds are winning new converts, it gives aid and comfort to the pedlars of nonsense. The forces of the new Counter-Enlightenment may seem an incongruous coalition – latter-day hippies and medieval theocrats, New Age and Old Testament – but they have been remarkably effective; so much so that we have now reached the point at which even a British prime minister who recites the mantra 'education, education, education' will defend the teaching of creationism in biology classes, with no apparent shame or embarrassment.

The sleep of reason brings forth monsters, and in the past two decades or so the spread of irrationalism has produced monsters galore. Some are manifestly sinister, others perhaps seem merely comical – "harmless fun", as Nancy Reagan said of her husband's reliance on astrology.

Cumulatively, however, the proliferation of obscurantist bunkum and the reaction against reason are a menace to civilisation. Last spring, only days after backing the Gateshead goons, the prime minister delivered a speech to the Royal Society extolling "proper science" and warning against "a retreat into a culture of unreason". Surprisingly, none of the assembled scientists shouted out "physician, heal thyself". Perhaps they feared that he would take it literally, and reach for one of Cherie's magic crystals.