An undercover investigation has revealed disturbing evidence of Islamic extremism at a number of Britain's leading mosques and Muslim institutions, including an organisation praised by the Prime Minister.

Secret video footage reveals Muslim preachers exhorting followers to prepare for jihad, to hit girls for not wearing the hijab, and to create a 'state within a state'. Many of the preachers are linked to the Wahhabi strain of Islam practised in Saudi Arabia, which funds a number of Britain's leading Islamic institutions.

A forthcoming Channel 4 Dispatches programme paints an alarming picture of how preachers in some of Britain's most moderate mosques are urging followers to reject British laws in favour of those of Islam. Leaders of the mosques have expressed concern at the preachers' activities, saying they were unaware such views were being disseminated.

At the Sparkbrook mosque, run by UK Islamic Mission (UKIM), an organisation that maintains 45 mosques in Britain and which Tony Blair has said 'is extremely valued by the government for its multi-faith and multicultural activities', a preacher is captured on film praising the Taliban. In response to the news that a British Muslim solider was killed fighting the Taliban, the speaker declares: 'The hero of Islam is the one who separated his head from his shoulders.'

Another speaker says Muslims cannot accept the rule of non-Muslims. 'You cannot accept the rule of the kaffir [non-Muslim],' a preacher, Dr Ijaz Mian, tells a meeting held within the mosque. 'We have to rule ourselves and we have to rule the others.'

The 12-month investigation also recorded a deputy headmaster of an Islamic high school in Birmingham telling a conference at the Sparkbrook mosque that he disagrees with using the word democracy. 'They should call it ... kuffrocracy, that's their plan. It's the hidden cancerous aim of these people.' The Darul Uloom school said it no longer employed the teacher and that one of the reasons he resigned 'was the incompatibility of many of his opinions with the policies of the school'.

When contacted by The Observer, UKIM said: 'We are a nationwide organisation and hold different programmes in our mosques. We are very concerned about this. We have instructed all our branches not to allow any more speakers with radical or fundamentalist views. This has occurred as a result of an internal problem. We hired out Sparkbrook community hall, and some of the organisations that hired it allowed some speakers with views that are not our own. As a result, no more external groups will be allowed to hire the community hall at Sparkbrook.'

Elsewhere the documentary records the huge popularity of DVDs and internet broadcasts produced by extremist preachers. At the Islamic bookstore at Regent's Park Mosque in central London, DVDs of a preacher called Sheikh Yasin are sold. In one DVD, Yasin, who is promoted on the mosque's website, accuses missionaries from the World Health Organisation and Christian groups of putting the 'Aids virus' in the medicine of African people, 'which is a conspiracy'.

Another DVD on sale features Sheikh Feiz, a Saudi-trained preacher. Feiz says: 'Kaffir is the worst word that can ever be written, a sign of infidelity, disbelief, filth, a sign of dirt.'

In a statement the company that runs the bookstore said: 'We sell and supply a wide range of material and we do not necessarily agree with it. It is totally unfair to blame [us] for any of the views expressed in these lectures.'

Elsewhere, another preacher at a mosque in the East Midlands is caught on film, praying: 'God help us in our fight against the kaffir, in every field, in every department of life. We beg you to help us fight against the enemies of our religion.'

Inside the Green Lane mosque in Birmingham, a preacher is recorded saying: 'Allah has created the woman deficient.' A satellite broadcast from the Grand Mufti of Saudi Arabia, Sheikh Abdul Aziz al-Sheikh, beamed into the Green Lane mosque suggests that Muslim children should be hit if they don't pray: 'When he is seven, tell him to go and pray, and start hitting them when they are 10.' Another preacher is heard saying that if a girl 'doesn't wear hijab, we hit her'.

Another preacher says: 'The time is fast approaching where the tables are going to turn and the Muslims are going to be in the position of being uppermost in strength and, when that happens, people won't get killed - unjustly.'

In a statement to Channel 4, Lord Ahmed, the convener of the government's Preventing Extremism taskforce, said he was worried about the programme's consequences: 'While I appreciate that exaggerated opinions make good TV, they do not make for good community relations.'

A spokesman for Green Lane mosque said Islam does not denigrate women and that the instruction to hit a child was merely a smack. He accused C4 of intensifying the 'witch-hunt' against Muslims.

'Undercover Mosques', Dispatches, goes out at 8pm on Monday, 15 January