The paternal grandfather of a Jewish 10-year-old who was given a judge's permission to be baptised has spoken for the first time to reveal his family's sorrow and criticise the judge for the decision.

Judge John Platt told the child, who cannot be named for legal reasons, that she should be "baptised... as soon as your minister feels you are ready".

His ruling backed the girl's father, a recent Christian convert who told his own father that he had found his new religion during a spiritual experience. The girl's mother applied to the courts after overhearing the girl "whispering to her dad about baptism".

"She feared she was going to be presented with a fait accompli," said the grandfather, who sees his grandchildren regularly and thinks of their mother (his ex-daughter-in-law) like a daughter.

Contrary to reports, the father came from an involved Jewish background, attending synagogue in his youth, while the mother came from a kosher home.

"My son insisted on a Jewish marriage and when his son was born he immediately arranged a brit" said the grandfather, who has been involved in his local synagogue. "But in later years he would more or less mock the religion." As a result, the girl and her brother were given little exposure to Jewish life. While the mother would have happily joined her in-laws on Friday nights or festivals, she was overruled by her husband.

"She loves Jewish things but she was very controlled by my son," said the grandfather. "She hasn't got that strength in her."

Soon after the couple divorced, his son came to him and spoke of a woman who had been miraculously cured of her afflictions. "He told us they were in a car and that suddenly Jesus was there too." The son said God had told him to go to Africa and build a church, which he is planning to do.

"It's the extent of his conversion that has surprised me," said the grandfather, whose son is now estranged from the family. "He has pushed us and all his old friends away."

The grandfather wishes the children's mother had put her foot down when the father demanded to take them to church. "There's no question she should have - she recognises that. The judge asked, why didn't she act earlier? But she's been dominated by my son for so many years, she's under his spell."

He believes she "let herself be walked over" in court, while the father was able to charm the judge.

"He showed his fervour and the judge must have thought this is someone extremely religious," he said. "I don't think he got a feel of what had gone on in. He asked why the mother didn't go to the priest to discuss the baptism before she applied to the court, but why didn't the father and the priest go to her before they did anything about baptism?"

Before the ruling, the judge was given a report by CafCass (the Children and Family Court Advisory and Support Service) on the child's wishes. The grandfather said he was surprised that the CafCass recommendation, to wait until she was older, was disregarded. "She's outgoing, but it's a huge step to assume she is capable of making this decision.

"She said to her mum, I'm really worried because you don't believe in Jesus, so you're going to hell. It's worrying because there is a possibility that that's the reason she wants to get baptised - that if she doesn't, she'll go to hell."

The girl has not yet been baptised, but is preparing to be. "I'm not saying she shouldn't convert if she wishes, just that she should be of an age when she has real maturity," said the grandfather.

He questioned why the judge only sought a written statement from a rabbi, and did not summon him for questioning. "If you don't know about Judaism surely you would seek advice?

Although under Israel's Law of Return the girl would encounter obstacles should she want to make aliyah, according to rabbis, she will always be Jewish. "It's a tragic story, but it is far from over. Her mum and her grandparents should not stop thinking of her as a Jewish child," said Rabbi Naftali Brawer. He said that the consequences would be more far-reaching if she had made the decision as an adult. "She's 10, and things may change."

The family still hope to reconcile with the son they love dearly, and in the meantime are involving the children in the Jewish community and looking ahead to the possibility of the girl marking her batmitzvah. The grandfather said. "That is going to be the next issue, I have no idea what will be the outcome. We can only look forward and show the children the positive side of being Jewish. If they then want to go down other roads, that's their choice."