A survey of 11,000 children found 22% of the girls and 9% of the boys said they had hurt themselves on purpose in the year prior to the questionnaire.
Rates of self-harm were worst (46%) among those who were attracted to people of the same or both genders.
The Children's Society report said gender stereotypes and worries about looks were contributing to unhappiness.
The self-harm statistics are included in the charity's annual Good Childhood Report, which examines the state of children's wellbeing in the UK.
The data on self-harm was analysed by The Children's Society after being collected in 2015 in the Millennium Cohort Study, a continuing research project following the lives of 19,000 children born in the UK between 2000 and 2001.
More than 11,000 of these children answered a questionnaire about whether they had hurt themselves on purpose in any way in the past year. Out of the 5,624 girls who responded, 1,237 said they had self-harmed. Continue reading.
Originally posted on BBC Health.