A survey of 2,300 people aged 10 to 17 found 58 per cent had asked for help because of concerns about their feelings, while 8 per cent indicated they had not sought help even though they did have worries.
Applying their results to 2018 mid-year population estimates by the Office for National Statistics, The Children's Society estimated that more than 464,000 children of this age in England, Scotland and Wales may have worries about their feelings and behaviour but have not sought help.
The charity warned that the mental health of these children could be at risk of deteriorating.
The research study found that of those who asked for help, three in four (77 per cent) said they had spoken to close family such as siblings or parents, followed by 38 per cent who were helped by friends and 36 per cent who were supported by an adult at school such as a teacher.
The data also showed that emergency services (police and fire services) and mental health charities saw a low percentage of children turning to them for help.
A 17-year-old participant told the charity: "I used to self-harm before, and it was like a way of me getting out the emotions because I never used to ask people for help...I thought it was something wrong with me so like I wouldn't want to ask for help."
The national charity said it believed a "fear of stigma and judgment" and a lack of knowledge about both mental health and the support services available were some of the "many" reasons that could contribute to young people not wanting to ask for help with their mental health.
Mark Russell, chief executive of The Children's Society, said: "The voices of children matter and if any child is having worries about their feelings or behaviour they should not have to suffer in silence. It's a travesty that such vast numbers of children who clearly need help are not telling anyone at all.
"We know that despite progress to break down stigma it is still a major barrier for young people. There could also be many other reasons that children aren't seeking help due to practical issues like nowhere they can go locally or no transport."
Earlier this year, the charity said it had found that around 110,000 10 to 17-year-olds were being turned away from mental health services each year because their problems were not deemed "serious" enough.
"It's vital children get help at an early stage to stop issues from spiralling," said Mr Russell.
"The next government needs to prioritise access to immediate emotional and mental health support for all children through their school or in their community so they can drop-in and chat when they need to."