The number of Britons choosing to become Muslims has nearly doubled in the past decade, according to one of the most comprehensive attempts to estimate how many people have embraced Islam.

Following the global spread of violent Islamism, British Muslims have faced more scrutiny, criticism and analysis than any other religious community. Yet, despite the often negative portrayal of Islam, thousands of Britons are adopting the religion every year.

Estimating the number of converts living in Britain has always been difficult because census data does not differentiate between whether a religious person has adopted a new faith or was born into it. Previous estimates have placed the number of Muslim converts in the UK at between 14,000 and 25,000.

But a new study by the inter-faith think-tank Faith Matters suggests the real figure could be as high as 100,000, with as many as 5,000 new conversions nationwide each year.

By using data from the Scottish 2001 census – the only survey to ask respondents what their religion was at birth as well as at the time of the survey – researchers broke down what proportion of Muslim converts there were by ethnicity and then extrapolated the figures for Britain as a whole.

In all they estimated that there were 60,699 converts living in Britain in 2001. With no new census planned until next year, researchers polled mosques in London to try to calculate how many conversions take place a year. The results gave a figure of 1,400 conversions in the capital in the past 12 months which, when extrapolated nationwide, would mean approximately 5,200 people adopting Islam every year. The figures are comparable with studies in Germany and France which found that there were around 4,000 conversions a year.

Fiyaz Mughal, director of Faith Matters, admitted that coming up with a reliable estimate of the number of converts to Islam was notoriously difficult. "This report is the best intellectual 'guestimate' using census numbers, local authority data and polling from mosques," he said. "Either way few people doubt that the number adopting Islam in the UK has risen dramatically in the past 10 years."

Asked why people were converting in such large numbers he replied: "I think there is definitely a relationship between conversions being on the increase and the prominence of Islam in the public domain. People are interested in finding out what Islam is all about and when they do that they go in different directions. Most shrug their shoulders and return to their lives but some will inevitably end up liking what they discover and will convert."

Batool al-Toma, an Irish born convert to Islam of 25 years who works at the Islamic Foundation and runs the New Muslims Project, one of the earliest groups set up specifically to help converts, said she believed the new figures were "a little on the high side".

"My guess would be the real figure is somewhere in between previous estimates, which were too low, and this latest one," she said. "I definitely think there has been a noticeable increase in the number of converts in recent years. The media often tries to pinpoint specifics but the reasons are as varied as the converts themselves."

Inayat Bunglawala, founder of Muslims4UK, which promotes active Muslim engagement in British society, said the figures were "not implausible".

"It would mean that around one in 600 Britons is a convert to the faith," he said. "Islam is a missionary religion and many Muslim organisations and particularly university students' Islamic societies have active outreach programmes designed to remove popular misconceptions about the faith."

The report by Faith Matters also studied the way converts were portrayed by the media and found that while 32 per cent of articles on Islam published since 2001 were linked to terrorism or extremism, the figure jumped to 62 per cent with converts.

Earlier this month, for example, it was reported that two converts to Islam who used the noms de guerre Abu Bakr and Mansoor Ahmed were killed in a CIA drone strike in an area of Pakistan with a strong al-Qa'ida presence.

"Converts who become extremists or terrorists are, of course, a legitimate story," said Mr Mughal. "But my worry is that the saturation of such stories risks equating all Muslim converts with being some sort of problem when the vast majority are not". Catherine Heseltine, a 31-year-old convert to Islam, made history earlier this year when she became the first female convert to be elected the head of a British Muslim organisation – the Muslim Public Affairs Committee. "Among certain sections of society, there is a deep mistrust of converts," she said. "There's a feeling that the one thing worse than a Muslim is a convert because they're perceived as going over the other side. Overall, though, I think conversions arouse more curiosity than hostility."

How to become a Muslim

Islam is one of the easiest religions to convert to. Technically, all a person needs to do is recite the Shahada, the formal declaration of faith, which states: "There is no God but Allah and Mohamed is his Prophet." A single honest recitation is all that is needed to become a Muslim, but most converts choose to do so in front of at least two witnesses, one being an imam.

Converts to Islam

Hana Tajima, 23, fashion designer

Hana Tajima converted to Islam when she was 17. Frustrated by the lack of variety in Islamic clothing for converts she founded Maysaa, a fashion house that designs western-inspired clothing that conforms to hijab.

"It's true that I never decided to convert to Islam, nor was there a defining moment where I realised I wanted to be Muslim. My family aren't particularly religious. I was interested in religion, but very disinterested in how it related to my life. I grew up in rural Devon where my Japanese father was the ethnic diversity of the village. It wasn't until I studied at college that I met people who weren't of the exact same background, into Jeff Buckley, underground hip-hop, drinking, and getting high. I met and became friends with a few Muslims in college, and was slightly affronted and curious at their lack of wanting to go out to clubs or socialise in that sense. I think it was just the shock of it, like, how can you not want to go out, in this day and age.

"It was at about that time that I started to study philosophy, and without sounding too much like I dyed my hair black and wore my fringe in front of my face, I began to get confused about my life. I was pretty popular, had good friends, boyfriends, I had everything I was supposed to have, but still I felt like 'is that it?' So these things all happened simultaneously, I read more about religion, learned more about friends of other backgrounds, had a quarter life crisis. There were things that drew me to Islam in particular, it wasn't like I was reaching for whatever was there. The fact that the Qur'an is the same now as it ever was means there's always a reference point. The issues of women's rights were shockingly contemporary. The more I read, the more I found myself agreeing with the ideas behind it and I could see why Islam coloured the lives of my Muslim friends. It made sense, really, I didn't and still don't want to be Muslim, but there came a point where I couldn't say that I wasn't Muslim.

"Telling my family was the easy part. I knew they'd be happy as long as I was happy, and they could see that it was an incredibly positive thing. My friends went one of two ways, met with a lack of any reaction and lost to the social scene, or interested and supportive. More the former, less the latter."

Denise Horsley, 26, dance teacher

Denise Horsley lives in North London. She converted to Islam last year and is planning to marry her Muslim boyfriend next year.

"I was introduced to Islam by my boyfriend Naushad. A lot of people ask whether I converted because of him but actually he had nothing to do with it. I was interested in his faith but I went on my own journey to discover more about religion.

"I bought loads of books on all the different religions but I kept coming back to Islam - there was something about it that just made sense, it seemed to answer all the questions I had.

"I would spend hours in the library at Regents Park Mosque reading up on everything from women's rights to food. Before I went to prayers for the first time I remember sitting in my car frantically looking up how to pray on my Blackberry. I was so sure people would know straight away that I wasn't a Muslim but if they did no-one seemed to care.

"During Ramadan I'd sit and listen to the Qur'anic recitations and would be filled with such happiness and warmth. One day I decided there and then to take my shahada. I walked down to the reception and said I was ready to convert, it was as simple as that.

"My friends and family were rather shocked, I think they expected there would be some sort of huge baptism ceremony but they were very supportive of my decision. I think they were just pleased to see me happy and caring about something so passionately.

"I grew up Christian and went to a Catholic school. Islam to me seemed to be a natural extension of Christianity. The Qur'an is filled with information about Jesus, Mary, the angels and the Torah. It's part of a natural transition.

"I do now wear a headscarf but it wasn't something I adopted straightaway. Hijab is such an important concept in Islam but it's not just about clothing. It's about being modest in everything you do. I started dressing more modestly - forgoing low cut tops and short skirts - but before I donned a headscarf I had to make sure I was comfortable on the inside before turning my attention to the outside. Now I feel completely protected in my headscarf. People treat you with a new level of respect, they judge you by your words and your deeds, not how you look. It's the kind of respect every dad wants for their daughter.

"There have been some problems. Immediately after converting I isolated myself a bit, which I now recognise was a mistake and not what Islam teaches. I remember a lady on a bus who got really angry and abusive when she found out I had converted. I also noticed quite a few friends stopped calling. I think they just got tired of hearing me say no - no to going clubbing, no to going down the pub.

"But my good friends embraced it. They simply found other things to do when I was around. Ultimately I'm still exactly the same person apart from the fact that I don't drink, don't eat pork and pray five times a day. Other than that I'm still Denise."

Daoud, 23

Daoud was a self-confirmed "racist" two years ago who knew nothing about Islam and supported the BNP. Now a Muslim, he describes himself as a Salafi - the deeply socially conservative and ultra-orthodox sect of Islam whose followers try to live exactly like the Prophet did.

"I was very ignorant to Islam for most of my life and then I went on holiday to Morocco, which was the first time I was exposed to Muslims. I was literally a racist before Morocco and by the time I was flying home on the plane a week later, I had already decided to become a Muslim."

"I realised Islam is not a foreign religion, but had a lot of similarities with what I already believed. When I came back home to Somerset, I spent three months trying to find local Muslims, but there wasn't even a mosque in my town. I eventually met Sufi Muslims who took me to Cyprus to convert.

"When I came back, I was finding out a lot of what they were saying was contradictory to what it said in the Qur'an. I wasn't finding them very authentic, to be honest. I went to London and became involved with Hizb-ut-Tahrir, the political group who call for the establishment of an Islamic state.

"But while I believe in the benefits of Sharia law, I left this group as well. The problem was it was too into politics and not as concerned with practicing the religion. For me, it is about keeping an Islamic appearance and studying hard. I think we do need an Islamic state, but the way to achieve it is not through political activism or fighting. Allah doesn't change the situation of people until they see what's within themselves.

"I have a big dislike for culture in Islamic communities, when it means bringing new things into the religion, such as polytheism or encouraging music and dance. There is something pure about Salafi Muslims; we take every word of the Qur'an for truth. I have definitely found the right path. I also met my wife through the community and we are expecting our first child next year."

Paul Martin, 27

Paul Martin was just a student when he decided to convert to Islam in an ice-cream shop in Manchester four years ago. Bored of what he saw as the hedonistic lifestyle of many of his friends at university and attracted to what he calls "Islam's emphasis on seeking knowledge," he says a one-off meeting with an older Muslim changed his life.

"I liked the way the Muslims students I knew conducted themselves. It's nice to think about people having one partner for life and not doing anything harmful to their body. I just preferred the Islamic lifestyle and from there I looked into the Qur'an. I was amazed to see Islam's big emphasis on science.

"Then I was introduced by a Muslim friend to a doctor who was a few years older than me. We went for a coffee and then a few weeks later for an ice cream. It was there that I said I would like to be a Muslim. I made my shahada right there, in the ice cream shop. I know some people like to be all formal and do it in a mosque, but for me religion is not a physical thing, it is what is in your heart.

"I hadn't been to a mosque before I became a Muslim. Sometimes it can be bit daunting, I mean I don't really fit into this criteria of a Muslim person. But there is nothing to say you can't be a British Muslim who wears jeans and a shirt and a jacket. Now in my mosque in Leeds, many different languages are spoken and there are lots of converts.

"With my family, it was gradual. I didn't just come home and say I was a Muslim. There was a long process before I converted where I wouldn't eat pork and I wouldn't drink. Now, we still have Sunday dinner together, we just buy a joint of lamb that is halal.

"If someone at college had said to me 'You are going to be a Muslim', I would not in a million years have believed it. It would have been too far-fetched. But now I have just come back from Hajj - the pilgrimage Muslims make to Mecca."

Stuart Mee, 46

Stuart Mee is a divorced civil servant who describes himself as a "middle-of-the-road Muslim." Having converted to Islam last year after talking with Muslim colleagues at work, he says Islam offers him a sense of community he feels is missing in much of Britain today.

"Everything is so consumer-driven here, there are always adverts pushing you to buy the next thing. I knew there must be something longer term and always admired the sense of contentment within my colleagues' lives, their sense of peace and calmness. It was just one of those things that happened - we talked, I read books and I related to it.

"I emailed the Imam at London Central Mosque and effectively had a 15 minute interview with him. It was about making sure that this was the right thing for me, that I was doing it at the right time. He wanted to make sure I was committed. It is a life changing decision.

"It is surprisingly easy, the process of converting. You do your shahada, which is the declaration of your faith. You say that in front of two witnesses and then you think, 'What do I do next?' I went to an Islamic bookstore and bought a child's book on how to pray. I followed that because, in Islamic terms, I was basically one month old.

"I went to a local mosque in Reading and expected someone to stop me say, 'Are you a Muslim?' but it didn't happen. It was just automatic acceptance. You can have all the trappings of being a Muslim - the beard and the bits and pieces that go with it, but Islam spreads over such a wide area and people have different styles, clothes and approaches to life.

"Provided I am working within Islamic values, I see no need in changing my name and I don't have any intention of doing it. Islam has bought peace, stability, and comfort to my life. It has helped me identify just what is important to me. That can only be a good thing."

Khadijah Roebuck, 48

Khadijah Roebuck was born Tracey Roebuck into a Christian family. She was married for twenty five years and attended church with her children every week while they lived at home. Now, divorced and having practiced Islam for the last six months, she says she is still not sure what motivated her to make such a big change to her life.

"I know it sounds odd, but one day I was Tracey the Christian and the next day I was Khadijah the Muslim, it just seemed right. The only thing I knew about Muslims before was that they didn't drink alcohol and they didn't eat pork.

"I remember the first time I drove up to the mosque. It was so funny; I was in my sports car and had the music blaring. I wasn't sure if I was even allowed to go in but I asked to speak to the man in charge, I didn't even know he was called an Imam. Now I wear a hijab and pray five times a day.

"My son at first was horrified, he just couldn't believe it. It's been especially hard for my mum, who is Roman Catholic and doesn't accept it at all. But the main thing I feel is a sense of peace, which I never found with the Church, which is interesting. Through Ramadan, I absolutely loved every second. On the last day, I even cried.