Under section 31(2) of the Children Act 1989, a court can only make a care or supervision order if there is evidence on the balance of probabilities that a child has suffered or is likely to suffer ‘significant harm’. ‘Harm’ is defined by section 31(9) of the Children Act 1989 and means ‘ill-treatment or the impairment of health or development including, for example, impairment suffered from seeing or hearing the ill-treatment of another.’ This definition includes sexual and emotional abuse.
‘Significant’ means the harm has to be serious enough to justify state intervention in a family’s life. Whether harm can be found to be ‘significant’ or not is going to depend on the facts of each particular case.
Lady Hale, in Re B (A Child)  UKSC 33 explains that significant harm must be ‘considerable, noteworthy or important,’ that it should be attributable to ‘a lack or likely lack of reasonable parental care, not simply to the characters and personalities of both the child and her parent,’ and finally: ‘where harm has not yet been suffered, the degree of likelihood that it will be suffered in the future must be considered.’
‘A systematic review of analysing models of significant harm’ Research from 2012.