Almost one million children are trapped in overcrowded living conditions in England, according to the housing charity Shelter.
It says official figures show 955,000 youngsters are living in "cramped, squalid" housing - a rise of 50,000 on three years ago.
The charity is calling on ministers to update current legislation on overcrowding, which does not take into account infants under one year old.
At present living rooms and even kitchens can count as bedrooms.
Shelter believes changing the definition of overcrowding would help to tackle the crisis.
It advocates a return to the approach in the 1930s when children between one and 10 were counted as "half a person".
The charity says overcrowding can cause depression and ill-health, and the lack of space makes it difficult for children to study, play and develop normally.
The Housing Act 2004 set out powers to update the statutory definition of overcrowding, but it has still not been changed.
Shelter has launched an online advertising campaign online to highlight the issue.