One of Islam’s most secretive sects has begun a charm offensive to win support for plans to build Britain’s biggest place of worship.

Tablighi Jamaat, an ultra-orthodox religious movement, abandoned its traditional dislike of modernity and politics to set up a website, broadcast on YouTube and hire a Westminster lobbying company.

The group, which intelligence agencies fear is a gateway to extremism, wants to build a mosque with a capacity for 12,000 worshippers. It would cost up to £75 million. Outline proposals for the site, beside the London 2012 Olympic Park, also envisage a religious boarding school with places for 500 pupils.

If the scheme goes ahead the new mosque would dwarf other mosques and churches; Britain’s largest Anglican cathedral, in Liverpool, holds a congregation of 3,000.

Tablighi Jamaat’s plans have been in a state of flux for months. Strong local opposition has generated bad publicity about a “mega-mosque” and there have been reports that government ministers will seek to block planning permission. The group claims that it has dismissed Ali Mangera, the award-winning architect who drew up futuristic designs for the mosque complex, and is revising its plans. But Mr Mangera told The Times he was still involved in the project and expected an announcement soon.

To improve its image Tablighi Jamaat has hired Indigo Public Affairs, a company that specialises in major planning battles. Indigo lobbies councillors and other planning authorities, organises community consultation exercises and liaises with the media.

Indigo has put Tablighi Jamaat on YouTube with a short statement from an unnamed representative who tries to calm fears about the size of the mosque and says that it will reach its 12,000 capacity only once or twice a year.

The lobbyists have also created a mosque website which claims that the Tablighi Jamaat movement “stands for democracy and freedom” and is “a role model to promote social and religious integration”.

Despite these claims, the site also adds that “complete success in this world and the hereafter is only achieved in following the way of life shown by Muhammad and every other way leads to failure in this world and the hereafter”.

Observers of the Islamic world portray Tablighi Jamaat as an extremely strict movement intent on spreading its version of the Muslim faith around the world. Gilles Kepel, an expert on Islam, said that the movement preached “a rigorous, unifying vision of Islam” which became “an advance guard for political Islamists”.

French intelligence and the FBI have aired concerns that jihadi groups could easily find recruits among Tablighi Jamaat’s younger adherents. The movement maintains that it “utterly refutes any links to terrorism or terrorists”.

Its new website states: “We are a mainstream, predominantly apolitical organisation seeking to go about our faith in a peaceful way. We do not teach an extremist line, but we clearly can’t speak for every single one of those who have ever attended our mosques. There are several thousand people at our weekly gatherings.”

Holy origins

— Tablighi Jamaat translates as “Proselytising Group”

— Founded in 1927 in India by Muhammad Ilyas

— 70-80 million followers in 150 countries

— Followers undertake missionary journeys on foot

— One million men attend annual gathering in Raiwind, Pakistan