A notorious "witch finder" who has been prevented by the Home Office from entering Britain is attempting to silence her critics with the threat of a £500 million lawsuit, it has emerged.

Helen Ukpabio, a born-again Christian preacher from Nigeria who heads up the controversial Liberty Foundation Gospel Ministries, is known for "deliverance sessions" to free children and adults from being possessed.

Among her advice to parents is the warning that children under two years old who cry in the night with a fever may be “possessed with black, red and vampire witchcraft spirits”.

The preacher, who calls herself "Lady Apostle", is threatening to sue a number of organisations, including the British Humanist Association (BHA) and the Witchcraft and Human Rights Information Network (WHRIN), for half a billion pounds in total.

She is accusing them of defamation, in part due to a difference in wording. Campaigners have accused Mrs Ukpabio, in her book Unveiling the Mysteries of Witchcraft, of writing “a child under two years of age that cries at night and deteriorates in health is an agent of Satan”.

But according to her solicitors, her actual words describe how such children can be possessed by “vampire witchcraft spirits” with signs of these being a child who “screams at night, cries, is always feverish, suddenly deteriorates in health, puts up an attitude of fear, and may not feed very well”.

Andrew Copson, BHA chief executive, commented: “The fact that she is threatening to launch a legal claim for half a billion pounds over an alleged distinction between being accused of exorcising "Satan" or "Vampires" tells you all you need to know about Ms Ukpabio.”

He added: “Given her baseless identification of features of "possessed children" and her dangerous and irresponsible teachings we feel a strong moral duty to point this out and will not be deflected by libel suits from wealthy "witch-finders".

In a letter sent to the BHA’s lawyers last Thursday, solicitors acting for the controversial preacher accused the charity of causing “members of the public to regard our client as an evil woman” with the damage to her reputation resulting in a “huge loss of incomes to her churches”. The letter, from London-based Graceland Solicitors, claims Mrs Ukpadio is “a Christian leader of international repute known and respected in many countries of the world”.

The Christian Preacher is known for "deliverance sessions" to free children and adults from being possessed (The New York Times) (New York Times)

Yet the Home Office recently revoked her visa after calls from campaigners earlier this year that she be banned from Britain on child protection grounds. It came after she held a number of services during a visit to London in April. Mrs Ukpabio subsequently complained to The Independent over its reporting.

Media lawyer Mark Stephens CBE of HowardKennedyFSI, who is acting for BHA and WHRIN, said: “We must remain ever vigilant of the danger of persecution of innocent children – babies, even – being branded as witches by latter-day self-styled witch-finders with perverse and pernicious views.” He added: “Freedom of speech is at its most precious when it permits voices to be raised against such evil. British libel laws must never be abused to censor matters of such public importance.”

And the concerns of British campaigners are shared by Nobel prize-winning Nigerian author, Professor Wole Soyinka.