Those being turned away include young people who are self-harming, have an eating disorder or have experienced abuse, according to analysis by the Education Policy Institute (EPI) of Freedom of Information (FOI) Act responses from CAMHS teams.
“Vulnerable young people in need of help are also facing a postcode lottery", says the annual report.
The analysis has been carried out by the EPI for the last four years and the proportion of those being denied treatment has not changed over that time.
While 17 per cent of referrals are rejected in London, the proportion rises to 28 per cent in the South of England, the Midlands and the East of England. Across England as a whole the average number of children and young people rejected for treatment is 26 per cent.
The most common reason is that young people’s conditions are seen as unsuitable for CAMHS specialists, do not meet their age specifications or are not eligible.
The EPI says that too often young people’s mental health problems are not considered serious enough by CAMHS services or their systems find it difficult to find suitable treatment options.
“These findings echo our previous research and raise concerns that the growing number of children and young people with complex needs that do not fit clearly into diagnostic boxes, those with lower-level mental health needs and older adolescents may be unable to access the support they require,” states the report.
Young people successfully accessing support are being seen quicker, with the median waiting times from referral to start of treatment falling by 11 days since 2015.
But they are still having to wait an average of two months to access support, double the four-week standard across England proposed by the government.
“This research shows that the system for providing mental health support to children is operating under great strain," said EPI senior researcher Whitney Crenna-Jennings.
“There is a vast treatment gap, meaning the needs of hundreds of thousands of young people in England are not being met.
“Denying children access to mental health treatment is likely to have major repercussions. As well as long-term health prospects, by hindering academic performance and development at an early age, mental ill-health also represents a key barrier to social mobility.”
She called on the government to focus on preventative support to ensure problems do not escalate in later life.
EPI chair and former Lib Dem coalition minister David Laws added: “Young people continue to be deprived of access to specialist mental health treatment, despite the government claiming significant investment in mental health services over the past five years.
“Progress in improving access over this period has been hugely disappointing, and it is unacceptable that as many as one in four children referred to mental health services are being turned away."
Councillor Judith Blake, chair of the Local Government Association’s children and young people board said: “This report reinforces the need to do more nationally to improve children’s mental health services and develop a system that says yes to children when they ask for or need help.
“Councils have a vital role in supporting children’s mental health and are keen to work with the government to promote early intervention and prevention so the system moves away from treating children once they are ill and moves further towards helping them and their families cope with the challenges they face earlier on.
“This includes long-term investment in the vital early intervention and prevention services which can help children avoid reaching crisis point in the first place.”
The NHS called the analysis “flawed” and claimed it is not an accurate reflection of “how modern services for families operate in partnership with other agencies”.
A spokesperson said: “The NHS is actually ahead of its target on ensuring as many children as possible receive mental health care – seeing an extra 53,000 children, teenagers and young adults last year, a 14 per cent increase on the year before and 22 per cent more staff in services than five years ago, against a backdrop of rising referrals.
“It is not the first time we have had to point out why the assumption that every referral should get NHS treatment when more appropriate support might be provided elsewhere – for example from schools and local authorities - is wrong.”
The Department of Health and Social Care has been contacted for comment.