‘When I look back, I suppose that Ben’s conversion to Christianity was quite gradual really, but he’s definitely fallen hook, line and sinker now,” says Ellen Parker, a teacher from Surrey and a lifelong agnostic. Her son Ben is in his mid-twenties and has been a devout evangelical Christian for a few years. It is a situation about which Parker feels deeply ambivalent.

“On the one hand,” she says, “he was quite aimless when he joined the church and I can see that it’s given him a real sense of purpose: he went from being someone who had literally never read a book for pleasure, to studying the Bible for hours each day. But it also makes me sad because none of the rest of the family shares his beliefs and it excludes us from a massive part of his life. Sometimes I think that the gulf between the values of his church and my own liberal values might be impossible to bridge.”

Parker is not alone. More and more baby boomer parents are watching as their children turn their backs on the secular, liberal values that they were brought up with, instead embracing Islam and evangelical Christianity. In the past few years at least 15,000 Britons are estimated to have made the shahadah (the formal declaration of faith in Allah and the prophet Muhammad), although many believe that this is an underestimation, given the numbers who convert in secret. And evangelical churches win far more converts than anywhere else in the Christian sphere.

Among those who have converted are the offspring of a number of Establishment figures, including Michael Howard’s son Nick, who worked with the homeless while studying at Oxford and is now training to become a priest, and Jonathan Birt, the son of John Birt, former director-general of the BBC, who has become a Muslim scholar and changed his name to Yahya.

Last week Lewis Wolpert, the scientist, wrote an article in the New Statesman referring to a conversation with Matthew, his evangelical Christian son. Matthew, he wrote, declared himself envious of his father.

“I beamed,” writes Wolpert, “and asked what he envied. The reply was, ‘You are going to die soon, certainly before me’. I was shocked. Why was this so desirable? It was because he was still unhappy and wanted to die so that he could go, as he strongly believed, to heaven.”

Wolpert nonetheless fully accepted his son’s religion. For others their child’s move towards religion creates a generational gulf as significant as that of the 1950s and 1960s, when rebellion was all about breaking boundaries. Now, it seems, the tables have thoroughly turned and there is a genuine thirst among many young people to establish strong moral codes and rules, to give up the alcohol, drug taking and casual sex synonymous with many of their parents’ western culture.

Myfanwy Franks, an author who has interviewed many British converts to Islam and Christianity, notes that “more and more it seems that becoming highly religious is the ultimate form of rebellion, because secularity is really our society’s main religion now. A lot of people utterly despise religion, don’t they? To convert to Islam or Christianity is really the punk rock of the modern age”.

The extent to which some parents worry about their child’s new-found faith cannot be underestimated. “When I was lecturing in Hull,” says Yvonne Ridley, the journalist who was captured by the Taliban in Afghanistan in 2001 before she converted to Islam, “I met a young man who had just become a Muslim and had brought his parents with him. He said to me, ‘I honestly think they would have found it easier if I’d become an alcoholic or a crack dealer’ .”

Part of this, of course, is borne of fear. Most parents want their children to have the easiest possible ride through life and, given the antipathy or even aggression that many people feel towards organised religion — and especially Islam — it is understandable that parents might be concerned about a child who converts. This applies even for older converts. Ridley’s mother is accepting of her faith but “on July 7 last year”, says Ridley, “she called me up and said, ‘Get that bloody hijab off and go home’. I didn’t take the hijab off, but I have to say that it was a really scary day to be out as a Muslim”.

Joe Ahmed-Dobson, son of the former health minister Frank Dobson, converted to Islam in his early twenties, a move that was immediately accepted by his parents. “I think people need to be sensible,” he says. “It’s probably best not to walk into the front room one day and say, out of the blue, ‘Hey, Mum and Dad, I’ve found religion!’ and I think maybe, subconsciously, that’s part of the reason why it took me so long to convert. I first read the Koran when I was 16 and I immediately accepted it philosophically, but I wanted my parents to notice that interest so that my decision didn’t come as a big shock to them.”

As Ahmed-Dobson sees it, it’s the secular world that is becoming outmoded, rather than the religious one. Every generation needs some faith and “for my parents’ generation”, he continues, “I think that they had a real belief in conventional politics and government: whether it was the socialism of the 1970s, or the conservative liberalism that came along later. There was a real sense back then that those movements would solve all the world’s ills, but they didn’t. I think it’s maybe as a result of that that young people are now more open to religion, and particularly Islam, which allows for science and logic”.