A children's charity is taking High Court action against the government over its claims that some protections of children in care are "myths".
The Article 39 charity is seeking a judicial review of Department for Education guidance to English councils responsible for vulnerable children.
The "myth-busting guide" suggests some duties around social worker visits, protections for missing children and care leavers' support can be cut back.
DfE lawyers approved the guide.
The guide, which was published last summer, came from the DfE's innovation unit, which encourages new ways of working in children's social care.
It covers the interpretation of legal protections for children in care, care leavers, children who are in custody on remand, and children who go missing or run away.
It comes at a time when local authorities are struggling to pay for support for children in need of protection, with a predicted £2bn shortfall in children's services budgets by 2020.
Release from some of these duties may save them money in the short term, but campaigners say they are important statutory protections which cover some of the most vulnerable children in society.
Last year, 50 organisations and social work experts wrote to children's minister Nadhim Zahawi with a detailed analysis of discrepancies between the guide and the statutory position.
They asked him to withdraw the guide, but he refused - saying neither the legislation nor the statutory guidance had changed.
Director of Article 39, Carolyne Willow, said: "This document overwrites key obligations within our children's social care system, which were crafted over many years and subject to detailed public consultations.
"The protections the guide presents as mythical exist in our legislation and statutory guidance because of the real needs of children and young people."
She added that nothing in the guidance was about giving children more support.
Legal experts at Simpson Millar, acting on behalf of the charity, lodged the High Court action this week.
It seeks to have the guidance quashed and removed from circulation.
Solicitor Oliver Studdert said: "In following this guide, local authorities will be denying fundamental support and protection, set out in law, to large numbers of children and young people."
He said it could lead to local authorities acting unlawfully and weaken the protections given to vulnerable children and young people.
The parts of the guidance being challenged include:
- Councils not having to offer a return home interview to children who have run away or gone missing
- Councils not having to appoint separate social workers to foster carers and children on long-term foster placements
- Allowing personal advisers to take on local authority duties in respect of young people who continue to live with their foster carers beyond the age of 18
- Reductions in the number of social worker visits to children in some foster placements
Enver Solomon, chief executive of the Just for Kids Law charity, said: "We know from our casework that local authorities all too often fail to meet their legal obligations to children and young people, and it should be the government's first priority to ensure that they do, rather than publishing misleading information that could result in even more unlawful behaviour on the part of statutory services."
The Department for Education would not comment directly on the case, but said it had received the application.
A spokesman for the Local Government Association said it supported giving experienced professionals the flexibility to try new approaches, as long is it is scrutinised and regulated.
He said the guidance had provided helpful advice on where councils could able to do things differently if they felt this was in the best interests of children.
And he added: "But it is important that any questions around the accuracy of elements of this advice are clarified as soon as possible, so that councils and their residents can be confident that any action taken is fully in line with current legislation and guidance."