A government decision to cap the age at which vulnerable children must be kept in suitable accommodation - away from tents, barges and caravans - is being challenged in the courts.
Article 39, a children's rights charity, has applied to the High Court for a judicial review of legislation which says children in care must always live in care settings - but only to the age of 15.
A change in the law was announced in February following a year-long consultation.
Article 39 said the arrangement, due to begin in September, will leave "thousands of children in care aged 16 and 17 without protection".
A Sky News investigation found at least 10,000 children had been placed in potentially unsafe accommodation including caravans, tents and barges.
Figures obtained under the Freedom of Information Act showed local councils used what is known as "unregulated accommodation" to house vulnerable children - even though those settings were not subject to inspection or regulation by Ofsted.
Between January 2019 and December 2020, at least 9,990 children were placed in unregulated accommodation by 86 local authorities. At least 20 children were sent to live in tents or caravans, 17 were placed in hostels, and seven were housed in barges on canals.
It is understood that Article 39 has mentioned the Sky News report in its High Court submission.
The charity said large numbers of 16 and 17-year-olds would potentially benefit if the age range was lifted, describing them as "similarly extremely vulnerable".
"The government's own data shows that only around 100 children aged 15 and younger will benefit from their legal change, whereas at any one time around 6,000 children aged 16 and 17 who are looked after by local authorities are living in unregulated accommodation," it said.
"A third of 16 and 17-year-olds in care currently live in unregulated accommodation. Around four in 10 children living in unregulated accommodation were put there by local councils within a week of entering care."
Article 39 said capping the policy at the age of 15 is "discriminatory and an injustice to older children in care".
Speaking in February, Education Secretary Gavin Williamson said: "Vulnerable children under 16 are too young for the type of accommodation that provides a place to stay but not the care and support that they need.
"The action taken today, supported by the sector and in response to their views, is an important step in making sure children in care are placed in settings that give them the highest chances of success."