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.

Since 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 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 study by the inter-faith think-tank Faith Matters suggests the real figure could be as high as 100,000, with as many as 5000 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 and then extrapolated the figures for Britain as a whole.

In all they estimated 60,699 converts lived 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 1400 conversions in the capital in the past 12 months which, when extrapolated nationwide, would mean about 5200 people adopting Islam every year.

The figures are comparable with studies in Germany and France which found there were about 4000 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 to help converts, 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.”

Catherine Heseltine, a 31-year-old convert to Islam, made history last 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 to the other side. Overall, though, I think conversions arouse more curiosity than hostility.”