Anjem Choudary tells them that registering their marriages is forbidden in Islam, as it would be recognition of British law.
But critics said he was leaving women open to abuse, as they could not go to a normal court to escape a violent husband or win their share in a divorce.
Mr Choudary’s Islam 4 UK group wants to carry 500 coffins through the Wiltshire town to remember Afghans killed by Britain and America.
He styles himself the judge of the UK Sharia Court and principal lecturer of the London School of Sharia and has claimed that he runs a network of Islamic courts in London, Luton, Birmingham, Derby and Leicester.
He claims that he has married more than 1,800 couples across Britain in less than ten years and conducted hundreds of divorces.
He spoke out to Douglas Murray, who has written a research paper on sharia law in the UK for the Charles Douglas Home Memorial Prize.
Mr Choudary, 42, did not give addresses for any of his sharia courts or his college.
Asked if he told newly-weds to register their weddings, he said: ‘No. Because once you’ve gone down that road of registering the marriage...you are automatically really saying, “Look, we are accepting the [non-Islamic] system that goes with it”.’
Last night women’s groups and moderate Muslims accused Mr Choudary of giving ‘dangerous’ advice to couples.
They said if marriages are not registered, men can take further wives.
Mr Murray said: ‘There are lots of very vulnerable women who have been deceived by this man. They are being told not to register their marriages so that they don’t have any rights as a wife in the eyes of the law in Britain.
‘The Government must not be frightened by accusations of Islamophobia and political correctness and ensure there is one law followed in Britain.’
Imam Shahid Raza, one of Britain’s leading sharia judges, said: ‘As far as I know, Mr Choudary does not have the right qualifications to qualify as a sharia judge.’ He said this ‘could not be condoned’.
Mr Choudary, estimated to claim £25,000 a year benefits, has written to dead soldiers’ families, saying he has ‘no sympathy for them whatsoever’.
This weekend he could not be reached for comment.