A woman who coached her three children to say "negative things" about their father without justification must have the youngsters taken from her care, a family court judge has ruled.
Judge Charles Atkins decided that the children must move to live with their father.
He said the mother was "very hostile and bitter" towards he ex-husband and was trying to undermine their relationship with him.
Her behaviour was causing the youngsters psychological damage, he said.
The judge outlined his decision in a written ruling after analysing evidence at a private hearing at a family court in Croydon, south London, last year.
He indicated that the children had stayed with the woman following a divorce but had regularly seen their father.
Social services bosses at Sutton Council in south London have welfare responsibility for the children and had raised concern about the effect the woman's behaviour was having.
The judge said the children, none of whom have yet reached their teens, could not be identified.
"I am afraid I am satisfied, and for whatever reason, (the woman) is very hostile and bitter towards the father of the children, and she makes this plain, even in front of the children," said Judge Atkins.
"She has nothing positive to say about him and many negative things."
He added: "I am satisfied that she does try to undermine his relationship with the children.
"She repeats false allegations to the children and those allegations are of a serious nature, such as wishing to poison them.
"She regularly coaches the children to say negative things about the father without any justification."
The judge said he had to decide what was best for the children.
"I have come to the conclusion that it is in the best interests of the children for them to be transferred to the care of their father," he said.
"I am satisfied that this is a mother that has caused, and is like to continue to cause, significant harm to the children, including by actually trying to undermine their relationship with the father."
He added: "I think her behaviour is emotionally abusive of them and psychologically damaging to them."
Judge Atkins said the woman would be able to stay in contact with the children and approved a plan put forward by social workers.
The judge said she was the children's "only mother" and should have a "good relationship" with them.
He added: "I think this father can be trusted to try to encourage a relationship between the children and their mother. It is very important for them.
"She is the only mother they have, she is the only mother they will ever have, and if they can have a good relationship with her that is vitally important, but I am satisfied that this father knows that. I think he will do his best to try to ensure that that will happen."