Thousands of children are growing up in "dangerous" converted shipping containers and office blocks in "cramped conditions", a report has found.
The Bleak Houses report by the Children's Commissioner for England estimates there could be more than 210,000 children without a permanent home in England.
It warns that temporary accommodation is frequently not fit to live in and children are spending years in interim housing while they wait for an offer of permanent accommodation.
Some 124,000 youngsters are classed as officially homeless and living in temporary accommodation - plus around 90,000 in "sofa-surfing" families - but the report believes that the actual figures could be much higher due to a lack of data.
Shipping containers are being re-purposed for use as temporary accommodation, the report found, leading to cramped conditions and shifting temperatures during the seasons.
Some parents were concerned about anti-social behaviour in the surrounding areas, forcing them to keep their children inside the small units instead.
Residents living in converted container accommodation in Ealing, west London, told the PA news agency the units were prone to damp and mould and overheating.
Office blocks and warehouses are also being used as temporary accommodation for families, with at least 13 office blocks in Harlow, Essex, converted into more than 1,000 individual flats.
Some units in Templefields House measure 18 square metres and are being used to house whole families, with parents and children sleeping in a single room also used as the kitchen, the report found.
Polly Neate, chief executive of housing and homelessness charity Shelter, called the analysis "damning" and said homelessness was "robbing... children of a decent childhood".
"No child should be spending months if not years living in a converted shipping container, a dodgy old office block or an emergency B&B," she said.
"But a cocktail of punitive welfare policies, a woeful lack of social homes and wildly expensive private rents mean this is frighteningly commonplace.
"We constantly hear from struggling families forced to accept unsuitable, and sometimes downright dangerous accommodation because they have nowhere else to go.
"The devastating impact this has on a child's development and wellbeing cannot be overstated."
Children and families told of the "deeply disruptive impact" moving away can have, with some 23,000 families in 2018 housed in temporary accommodation outside of their home council area.
The Children's Commissioner Anne Longfield also expressed concerns about B&Bs used as temporary accommodation, creating "intimidating and potentially unsafe environments" for children.
The bathroom in B&Bs is often shared with other residents and vulnerable adults, including those with mental health or drug abuse problems.
Of the 2,420 families known to be living in B&Bs in December 2018, a third had been there for more than six weeks - despite this being unlawful.
Analysis in the report, released on Wednesday, found that in 2017, around two in five children in temporary accommodation had been there for at least six months.
Around one in 20 - an estimated 6,000 children - had been there for at least a year.
The report warns that a further 375,000 children in England are in households that have fallen behind on rent or mortgage payments, putting them at financial risk of becoming homeless in the future.
Commenting on the report, Ms Longfield said: "Something has gone very wrong with our housing system when children are growing up in B&Bs, shipping containers and old office blocks.
"Children have told us of the disruptive and at times frightening impact this can have on their lives.
"It is a scandal that a country as prosperous as ours is leaving tens of thousands of families in temporary accommodation for long periods of time, or to sofa surf.
"It is essential that the Government invests properly in a major house-building programme and that it sets itself a formal target to reduce the number of children in temporary accommodation."
A Government spokesperson said: "No child should ever be without a roof over their head and we are working to ensure all families have a safe place to stay.
"If anyone believes they have been placed in unsuitable accommodation, we urge them to exercise their right to request a review.
"We have invested £1.2 billion to tackle all types of homelessness, including funding a team of specialist advisers which has, in two years, helped LAs to reduce the number of families in B&B accommodation for more than six weeks by 28%."