Children's rights charity Article 39, claims that Department for Education "statutory guidance myth busting", produced by its children's social care innovation programme, features "numerous errors and misrepresentations of the statutory framework" on how social workers should support children in care.
A key concern of the charity is that the DfE guidance suggests that it is acceptable for local authorities to provide one social worker for children and foster carers when a child is in a stable, long-term placement. According to the charity, the statutory guidance indicates that for safeguarding reasons, there should be two.
Article 39 director Carolyne Willow said that the charity had resorted to legal threats because it could not "stand by and leave it to vulnerable children to have to go to court to defend the rights that parliament and successive governments have given them".
The myth buster states: "Whilst the regulation states that there needs to be a qualified social worker for the child and the foster carer, it does not explicitly say that they need to have different social workers."
The DfE document also states that children who have run away should be offered an interview with someone independent after they return, yet the charity claims that there is a statutory obligation to offer this safeguard.
Article 39 said it had made a series of freedom of information requests which revealed that Ofsted has also questioned the DfE guidance.
The charity claimed that Ofsted told the DfE that "all local authorities to date" had interpreted the statutory guidance to mean two social workers were required - "one supporting the child and the other the foster carers".
Article 39 quotes Ofsted as stating that to reduce this to one would be "removing a significant safeguard for children in foster care".
The charity's proposed action follows a joint letter signed by 50 charities and social work experts sent to children's minister Nadhim Zahawi in September last year, setting out the legal inaccuracies in the document.
It urged Zahawi to withdraw parts of the document that it believes conflict with existing legislation and government guidance, "to avoid the risk of confusion and to prevent any harmful effects on vulnerable children and young people, and those caring for them".
Zahawi responded saying the myth buster did not seek to change the law, but to clarify misunderstood elements of statutory guidance. He added that it had the backing of local authority children's services leaders.
Willow said: "It is not good enough for the minister to say there have been no changes to the law and statutory guidance while at the same time leaving in circulation a document which indicates otherwise."
The DfE confirmed it had been contacted by lawyers regarding legal proceedings.
A spokesperson said: "We have received the pre-action letter and will be responding in due course."