Girls in Britain are becoming more miserable, suggests the Children's Society's annual report.
Among 10 to 15-year-old girls, the charity's report says 14% are unhappy with their lives as a whole, and 34% with their appearance.
Researchers were told of girls feeling ugly or worthless.
The figures for England, Wales and Scotland for 2013-14 represent a sharp rise in unhappiness on five years before.
By contrast the study found that boys' sense of happiness remained stable.
Children's Society and University of York researchers examined responses on the wellbeing of 10 to 15-year-olds.
They found that between 2009-10 and 2013-14 on average 11% of both boys and girls said they were unhappy.
But the latest available figures, for 2013-14, showed the proportion of girls saying they were unhappy had risen to 14%.
It follows research recently published by the Department for Education which showed the mental well-being of teenage girls in England has worsened, compared with their counterparts in 2005.
The study highlighted the growing pressure of social media and suggested that a tough economic climate had created a more "serious" generation of young people.
Lucy Capron from the Children's Society told BBC Radio 5 Live: "This isn't something which can be explained away by hormones or just the natural course of growing up, actually this is something that we need to take seriously and we need to address."
The proportion of girls reporting being worried about their looks rose from 30% for the period as a whole, to 34% in the year 2013-14 - while the proportion of boys unhappy with their appearance remained unchanged at 20%.
Under pressure: Teens speak out
Three girls tell BBC Radio 5 Live how they feel.
Megan, 12, said: "The only time that I'm not happy is if people are judging me or being mean and things like that. With people at school, they post things [on social media] and they try and make everyone think that they are perfect.
"Sometimes it makes me feel - not annoyed - but I don't want to look at it any more because they just do it all the time and it gets on your nerves."
Natalia, 15, said: "Everywhere you look it's like, celebrities: thin, blonde or - perfect teeth, perfect hair, perfect eyes, perfect eyebrows. And it's just crazy and I just feel like I should look like that - even though I know it's all like fake, or a lot of it is anyway.
"I have these days when I'm like, I don't care what people think but then somebody will say something and it will just hit me again and I'll feel worse but I don't know, it's hard to explain why it bothers me so much sometimes."
Caitlyn, 12, said: "I am happy most of the time, but then when it comes to my friends going: 'Ah I look really beautiful in this outfit' and everything, I just feel like, no, I can't do that - I can't pull it off.
"When I'm obviously looking through my Facebook and looking at some of the posts, all you can see is pictures of celebrities and my friends looking beautiful in selfies and everything, and then there's just me, like, I can't get away from any of it."
While teenage angst is nothing new, Ms Capron said: "What's new and what the Children's Society have unveiled is the scale of the problem - particularly the fact that the gap between boys and girls is getting wider and that's something that we should be worried about."
The reasons for the deteriorating picture for girls are not clear says the charity - but the report finds that emotional bullying, such as name-calling, is twice as common as physical bullying among boys.
The report also suggests that girls are more likely to spend extended periods on social media which has been linked to a higher risk of mental ill health.
Ms Capron said relationships, and the way they are played out on social media, are big drivers in a young person's life.
"Some other research has shown that girls are spending a lot more time on social media - up to three hours a night in some cases - and we need to make sure that's done in a safe way," she said.
In another study, childcare professionals have published evidence that children could be worrying about being fat or ugly at a younger age, with girls particularly affected.
The Professional Association of Childcare and Early Years says staff have noticed children as young as three being worried about their appearance.
Overall, nursery staff, childminders and nannies looking after under-10s in England, reported hearing children:
- expressing unhappiness with parts of their bodies and with their body size
- describing themselves or another child as fat
- saying they feel ugly or less good looking than someone else
- refusing food for fear it will make them fat
The risk is that these views could prompt eating disturbances and depression later in life, according to Middlesex University child development lecturer, Dr Jacqueline Harding.
She suggested that media images and adults chatting about diets could lead to negative body images in children.
Parents can help boost body confidence, for example by praising children for acts of kindness rather than for their looks, she advised.
The association is calling for more support and government guidance on these issues.