Almost one in three parents have sacrificed the biggest bedroom in the house to their children, according to a study into "space-starved" families.
Some 31% of adults living with children under the age of 18 said they have squeezed into a smaller room themselves in order to allow their offspring some extra space, the research from the Post Office found.
One fifth (21%) of families who were questioned said their children share bedrooms, and of these, almost half said their children have no other choice but to sleep in the same room as their siblings because space is too tight.
Children were most likely to be found sharing bedrooms at around the ages of four to six years old and least likely between the ages of 14 and 18.
More than half (58%) of parents wish their children had bigger rooms in which to study and play, and two-thirds (63%) aspire to trade up to a larger property, according to the UK-wide survey of more than 2,000 people.
The Post Office said its findings show the "struggle for room" among "space-starved" households.
It highlighted figures from the Royal Institution of Chartered Surveyors (Rics) that show the average size of a home has decreased "dramatically" over the last 90 years, from 1,647 square feet to 925 square feet.
John Willcock, head of mortgages at the Post Office, said: "With the average house size continuing to shrink, and with many family homes often lacking a garden, it's hardly surprising that so many of us are concerned about this lack of space and the impact it will have on our children.
"Unfortunately, the reality of the situation means a lot of families feel like they're letting their children down by not being able to provide them with their own room, or playroom where they can study or play.
"As a result a number of parents are making sacrifices and giving up larger rooms in the home to children."
Research released by property website Zoopla earlier this week found that 40% of homeowners believe that securing a mortgage is more difficult than it was three months ago.
Stricter mortgage lending rules came into force at the end of April under the Mortgage Market Review (MMR), which forces lenders to ask applicants looking to buy a home or remortgage more thorough questions about their borrowing habits.
There have also been predictions that eventual rises in interest rates in the coming years will leave more homeowners trapped with their current lender and unable to get a mortgage deal elsewhere.
Think tank the Resolution Foundation recently estimated that around 800,000 borrowers may become "mortgage prisoners" by around 2018 as interest rates rise, because tighter conditions mean they have no option but to stay on their existing lender's standard variable rate (SVR).
A separate report released earlier this week by charity Shelter highlighted the struggle for young adults to move out of the home they grew up in.
Shelter, which is urging stronger action to help the "clipped wing" generation fly the nest, found that nearly two million working young adults aged between 20 and 34 years old in England are still living with their parents.