A huge rise in the number of children calling to report sexual abuse by women has been revealed by ChildLine.
Over the past five years, the charity says the number of such calls has risen five times faster than youngsters reporting abuse by a man.
Of 16,094 children who called ChildLine about sex abuse last year, 2,142 told of abuse by a woman, up 132% on 2004-5.
Men still account for the majority of child abuse claims, but the NSPCC said female sex abuse was under-reported.
This is because there is a reluctance or unwillingness on the part of professionals to acknowledge or identify sexual abuse by females, the charity suggested.
The research follows the recent high-profile case of nursery worker Vanessa George, who abused children in her care. She was a member of an internet paedophile ring along with another woman.
Childline's report did not claim that sexual abuse by women is on the rise.
It instead suggested that, as more boys are tending to call its helpline, more cases are being reported.
The research said nearly two-thirds (1,311) of the claims it received about sex abuse by a female involved the child's mother.
Just over twice as many victims (2,972) said they had been abused by their father - which amounted to 45% of calls about sex abuse by males.
The number of children claiming to have been abused by men grew by 27% in the same four-year period.
The ChildLine research also showed that 42% more children were calling the helpline in 2008-9 than in 2004-5.
Sue Minto, head of ChildLine, said: "Most sex abuse calls to ChildLine come from girls saying they were assaulted by a male.
"But a growing number of callers now say they were sexually abused by a female. This may be partly because more boys are calling us than previously.
"Many would find it shocking that any woman - let alone a mother - can sexually assault a child. But they do."
Dr Lisa Bunting, senior researcher at the NSPCC, who has studied the issue, said: "There is such an intense stigma in disclosing incidents of abuse by women.
"We get a lot of stigma with any type of sex abuse, but this is particularly the case in the participation of women."
She said this often led to victims "internalising" the abuse because they could not believe it had happened and did not think they would be believed.
She added: "If you don't think females are capable of committing sex offences, then you are never going to be looking for that."
The ChildLine report said the issue of female sex offending was not well-reflected in policy, practice and guidance on child protection and offender management.
It added: "It is important that regardless of what is currently known about the numbers of female offenders, more is done to understand the nature of sexual offending by women [and to] raise awareness among the public so that they can report it."