The findings, published today in Children and Young People's Advocacy in England, raise concerns that many children are being denied access to these support services despite being entitled to them by law.
The report, published by children's commissioner for England Anne Longfield, is calling for councils to set out a clear strategy for their advocacy services, which are supposed to provide independent information and help to ensure children and young people are heard and their rights protected.
The strategy should show how they will be delivered, and work towards a "highly visible, easily accessible universal advocacy service for children and young people up to the age of 25".
"One impediment to the delivery of advocacy support to children from all eligible groups is confusion about their right to independent advocacy.
"The patchwork of legislation which both grants groups of children and young people the right to advocacy support, and confers the responsibility on local authorities to arrange this provision, is fragmented and unclear.
"Consolidating this legislation would be a positive first step in improving access across all eligible groups," the report says.
Researchers sent data requests to all directors of children's services in England to find out about provision in six eligible groups.
These include care leavers, children complaining about health services, children over 16 who lack mental capacity, children in mental health settings, children with special educational needs and disabilities and children in receipt of social care services who want to make a representation or complaint.
Councils were asked what arrangements were made where a legal duty existed and if they were provided in-house or by contract.
The results provided by 119 councils revealed a "large degree of variation" in councils' ability to report whether or not they provided services to different eligible groups.
The review finds that "too many services are inadequate: children and young people entitled to advocacy are not always able to access high-quality information, advice and support from advocates when they need it".
The rising number of children in care and children in need is further stretching services, it adds.
Despite all councils responding with details of how advocacy support was provided for care leavers, 29 per cent of areas said they did not know how complaints advocacy for children in receipt of health services was delivered.
"This is in spite of the transfer of this responsibility to local authorities from the Secretary of State for Health in 2012," the report says.
The data shows the highest demand for statutory advocacy was among care leavers, with an average of three referrals for every 10 young people.
"This means though, that the current level of provision sees (at most) 30 per cent of all eligible care leavers accessing advocacy services," the report concludes.
It says the research points towards a "significant" group of children being denied advocacy despite having a statutory entitlement to access it.
"In some local authorities, less than 75 per cent of care leavers' referrals are taken forward, despite the vulnerability of this group by virtue of the period of transition they find themselves in," the report added.
Children's rights charity Coram, which contributed to the report as part of a working group, backed the recommendations.
These include a call for training to be a mandatory requirement for all advocates as some lack the necessary experience to support children appropriately, the charity said.
It also welcomed the creation of a shared framework for measuring outcomes and impact of advocacy, co-produced by young people.
Brigid Robinson, managing director of Coram Voice, which operates the charity's advocacy support service, said: "Young people we consulted for the advocacy report told us that lack of information was a key issue.
"We support the call for more transparency and believe that requiring each local authority to publish their advocacy offer, in much the same way that they have to publish their ‘care leaver offer', would make it clearer to children and young people what advocacy is and how to get support if they need it."
Carolyne Willow, director of children's charity Article 39, urged the government to quickly accept and implement the recommendations.
Willow said: "Advocates empower children who are often in extremely powerless situations; they are not an optional extra but a crucial mechanism for making sure all children enjoy their rights."
Jenny Coles, ADCS vice president, said: "Children in care should be made aware of their rights, feel they are active participants in the decisions made about them and their lives, and there should be meaningful consideration of their views and wishes before, during and after decisions are made about their lives, such as where they will live.
"A wide range of professionals advocate for children in care and care leavers, from social workers and virtual school heads to independent reviewing officers and personal advisors, and children in care councils across the country give young people the chance to have a say about the things that matter to them and to shape and influence services for the better. Friends and family can act as advocates too. Children in care also have the right to make representations and complaints to the local authority regarding their care arrangements via independent advocacy.
"Thirty years on since the Children Act 1989 received Royal Assent it is helpful to remind ourselves of the key principles underpinning this legislation not only in relation to a child's right to advocacy, but also of our preventative duties which have never been sufficiently funded to enable us to work with children and their families earlier, addressing needs as and when they arise. This is compromising our abilities to improve their lives and life chances."