Puritanical yet wealthy, convinced of their God-given mission to the rest of the world, sure of a divinely inspired history - Saudi Arabia and the United States are surprisingly similar in their mixture of religion, politics and interference in other countries' affairs. Saudi Arabia has Wahhabi Islam, Middle America has evangelical Christianity. Historically, they hate each other. Yet both see themselves as exponents of the purest version of their faith. Both are suspicious of modernity. Both see no distinction between politics and religion.

Now these foreign spiritual empires are moving in on Britain. Increasingly, foreign-inspired and foreign-financed religious conservatives are influencing the UK political agenda, forming what amounts to a spiritual fifth column. Americans and Saudis are sending volunteers, establishing schools, setting up publishing houses and think-tanks, and taking over places of worship.

Whether it is opposing gay marriage, promoting family values in school, blocking scientific research on stem cells, opposing artistic and literary freedom of expression, or denouncing critical scholarship of sacred texts, the hardline Christians and the hardline Muslims find themselves on the same side. A new, cross-faith conservatism is in the air: witness how Catholics, Anglicans, Jews and Muslims supported Michael Howard's call for a cut in the time limit for abortion; how Muslims joined Christians in the unprecedented protests against the BBC's screening of Jerry Springer: the opera; how evangelical Christians supported the banning of a play offensive to Sikhs; how Jewish leaders opposed the BBC's cartoon series Popetown.

Politicians seem conscious of this spiritual awakening: Tony Blair panders to the moral minority, and has repeatedly invited faith leaders to No 10 to discuss the national agenda. To the intense frustration of the evolutionary biologist Richard Dawkins, even when confronted with schools teaching that creationism is as valid intellectually as Darwinism, the Prime Minister speaks only of "diversity". And for the first time, the leaders of all three of the UK's main political parties flew to Rome to attend a papal funeral. No prominent politician now feels like making the case for secular politics. Few people feel as strongly about secularism as the religious feel about their faith. The anti-religious vote is too small, too diffuse. And religious conservatives are swimming with the stream: even mainstream voters worry about the moral vacuum in schools, or the filth they see on television, at the theatre, or on the news-stands.

Some of this new movement is home-grown, but its values-based campaigning is boosted by foreign money and volunteers. Saudi-financed Islam is targeted at the voiceless, frustrated, jobless male youths of immigrant communities in big cities. Parents, grandparents and community elders watch in consternation as their sons (and infrequently daughters) lap up a rigid, censorious form of Islam, which includes the strict observance of prayer times, learning the Koran by rote, and a wholesale rejection of the habits, attitudes and values of mainstream society.

For example, the Muntada al-Islami Trust, a Saudi-funded "charity" headquartered in the UK, last year grossed £1.48m. According to Paul Marshall, a senior fellow at the Washington-based Centre for Religious Freedom, al-Muntada sees its mission as propagating a Wahhabist version of Islam. The trust owns al-Muntada Islamic primary school in Parsons Green, west London, with 198 students (both boys and girls), and it has also approached a number of independent Muslim schools with a view to buying them - although, thus far, it has acquired only the Eden High School in Hanworth, Middlesex. It has also started a fundraising initiative, the Muslim Education Fund, with the Hijrah School in Birmingham, a state-funded secondary school. All these schools, according to Idris Mears of the Association of Muslim Schools UK, are Ofsted-inspected. The next step for the Muntada Trust is to buy secondary schools.

Similarly, the King Fahd Academy in Acton, west London, is Saudi-funded and Wahhabi-influenced. Its 600 primary and secondary school children follow a Saudi curriculum. Former teachers and parents have criticised the academy's religious teaching for instilling "hostility to the outsider" and for discriminating against girl students. The academy devotes up to half of its lessons to religious education and has different curricula for boys and girls.

According to Navid Akhtar, a Muslim documentary-maker, many poorer Muslims accept the Saudi influence. "They see this Wahhabi or Arab-style Islam as purer. For the more self-confident (often second-generation) British Muslims - most of them from south Asia - Arab ideals can seem foreign and unacceptable. But the self-confidence of the Wahhabis often cowers their parents."

For the Christian missionaries, the targets are different: the unhappy and spiritually hungry members of a secular middle class that has forgotten, or never knew, the faith of past generations. This is the theology of the Alpha Course, of ten-point plans for salvation, of the idea that faith in God leads to worldly rewards. The moral certainties are the same: no sex, no drugs, a literal interpretation of sacred texts, male superiority over women, and a profound conviction that wider society is wallowing in sin.

Though there is no counterpart to the madrasa, to the dingy backstreet room with its videos of atrocities in Chechnya and Iraq, Christian evangelical training can be just as strident. Oral Roberts University in Tulsa, Oklahoma, set up by the televangelist after whom it is named, has trained ministers from the Christian Congress for Traditional Values - a UK-based pro-family, pro-life group - and from the Peniel Pentecostal Church, based in Pilgrims Hatch near Brentwood, Essex. Michael Reid, the founder and leader of that church, has imported, undiluted, the Oral Roberts message on social mores. According to Reid's (videotaped) sermons, gays "are filthy perverts", "Muslims are vile" and "God has placed fathers at the head of the household".

US Pentecostal and charismatic churches often send volunteers to help staff pro-life groups. The Kensington Temple evangelical church in west London regularly draws a 3,000-strong, racially mixed congregation. It has gained attention from critics and supporters alike because of its disciplined followers, who adopt the conventional Moral Majority views against homosexuals, adultery and abortion; and because of its regular visits from American evangelical preachers, with their fire-and-brimstone sermons. According to one former member: "You might have a midweek prayer meeting with a few hundred people where they invite a crusader to speak during the service."

At Wycliffe Hall, the Oxford evangelical theological training college (which last year played host to 74 undergraduates and 16 graduates), one American minister delivered a hardline pro-life talk in which he spoke of the blood of innocent children on society's hands, and kept repeating: "This is murder."

Some British groups are reluctant to admit US or Saudi links; others boast of their association with a more powerful and "theologically pure" patron. The Shirkatul Islamiyyah Trust, which registered a gross income of £2.5m in 2003, speaks openly of its Saudi funds, earmarked for "the propagation of Islam" through Muslim television and publications.

Among Christians, the former Tory leader Iain Duncan Smith and his Centre for Social Justice maintain close links with the conservative Heritage Foundation, Washington's biggest right-wing think-tank, which itself has close links to the Bush administration. Although the foundation would not describe itself as Christian, its agenda - pro family, pro life and pro American expansionism - fits neatly with the views of the Moral Majority. The foundation's Dr John Holsman says: "We work with Iain Duncan Smith and are ideologically in tune with the Conservative Party, but the relationship is informal." Similarly, while Joel Edwards of the Evangelical Alliance talks of his "tremendous debt" to the partnership the alliance enjoys with American groups, he stresses that it raises its own funds.

Liberal versions of Christianity and Islam are as much under threat from the foreign invaders as is British secularism. Moderate, mainstream British Christianity cannot compete with the glitzy style and high-pressure salesmanship of the American evangelists. Similarly, the poorly educated imams of Bradford and Tower Hamlets, ministering to believers who are barely a generation away from the village Islam of south Asia, lack the financial, theological and intellectual firepower to stand up to the missionaries for Saudi-style Islam. There are few followers in Britain of the "progressive Islam" now gaining ground in Canada and Australia, which attempts to reread the Koran in a non-literal way, and to ditch orthodox Muslim hang-ups on sex, clothing, diet and so on. One liberal imam, who regularly spoke out against the persecution of gay Muslims and the restrictions on girls' education, found himself hauled before the Muslim Council of Britain (which receives funding from Saudi Arabia) and told to explain himself. According to the gay rights activist Peter Tatchell, an insidious alliance has sprung up between ultra-orthodox Christians and Muslims whereby "the Christian Institute will snitch on liberal Muslims to the Islamic Council, which in turn has leaned on them, threatening their careers".

Saudi money has already had a pernicious effect on the intellectual climate in Britain. British universities find it very easy to raise money from Saudi donors for the study of uncontroversial subjects such as Islamic calligraphy, architecture or law. But pursue intellectual inquiry in the field of Koranic studies - making, in effect, the same theological journey that biblical scholars made in the 19th century - and your application for funds will be speedily turned down.

The Saudis may recognise that it is unlikely that new Labour will adopt sharia law, or that Britain will accept a theocracy; and the Americans may have no illusion about transforming a multi-faith society into a nation guided by the God of the Wasp. Yet campaigners from both Saudi Arabia and the US are convinced of their homeland's spiritual superiority and their duty to export its principles, civil and religious. Given the money, organisational infrastructure and proselytisers they boast, what can stop these two spiritual empires from turning Britain into one of their outposts? Reliance on the alleged national aversion for extremism and religious fervour seems a flimsy defence.

We have entered uncharted territory, with an ever-growing Muslim community politicised by events since 11 September 2001, and especially the UK government's draconian "prevention of terrorism" measures; and an ever more frustrated Christian community fed up with seeing its values trashed by the metropolitan liberal establishment. It is for the secular establishment to meet the challenge of stopping the attack on our way of life. It has to recognise that religion now identifies many people in the way race once did; that ignorance of religion is therefore dangerous; and that marginalising people of faith will simply push them towards extremists who are eager to take over them, and ultimately the rest of us.

Additional research by Samia Rahman