The early findings of a study comparing the experiences and views of teenagers in 1986 and 2006 reveal that the number who say they have no best friend they can confide in has risen from around one in eight to almost one in five.

The conclusion, submitted to the Good Childhood Inquiry, a two-year independent investigation of modern childhood overseen by the Children's Society, will prompt fresh concern over increased pressures and mental distress among children and young people in Britain.

The latest findings, from a YouthTrends study funded by the Nuffield Foundation, compare a 1986 survey of 16-year-olds tracked under the British Cohort Study with 700 of the same age group questioned in 2006. They reveal that the proportion who say they have no best friend they can trust has risen from 13% to 18%.

The change is being seen as significant by the Good Childhood inquiry team because friendship is so closely linked in a wide range of research to children's happiness. Submissions to the inquiry by children, published today along with other evidence on friendship, also reveal they rated friends more highly than any other factor when asked what makes a good childhood, and were deeply concerned about bullying.

Children wanted to be able to spend time with their friends, and regarded them as an important source of support.

Judy Dunn of the Institute of Psychiatry at King's College London and chair of the inquiry, said: "Adults often underestimate the importance of friendship for children, and how friends help them to adjust to school, the arrival of new siblings and the experience of being bullied."

Being separated from friends was often "a deeply unhappy experience for children" which could harm their mental health, she said, while being rejected by other children was closely linked to problems such as depressive moods and aggressive, antisocial and delinquent behaviour and a cycle of exclusion.

The apparent decline in close friendships among teenagers over the past two decades will add to concerns that the quality of childhood is also diminishing. Stephan Collishaw of the Institute of Psychiatry, author of the YouthTrends study, said the research was at an early stage, but described the findings as "worrying".

He said: "There is a lot of research showing that friendship is important for children's happiness. Separation from a best friend can contribute to poor mental health, and if children are experiencing stressful circumstances such as problems at school or home, having a best friend helps them cope and increases their resilience to overcome them."

An earlier study by the institute published in 2004 concluded that young people's wellbeing has declined over time, reflected in an increase in mental health problems, drug use and suicide. Researchers will now examine the relationship between the fall in close friendships and the decline in happiness.

The Good Childhood Inquiry, launched last autumn with backing from the Archbishop of Canterbury, Rowan Williams, is studying friendship as one of six themes, together with family, health, learning, lifestyle and values.