Older people should be encouraged to move into smaller homes to help tackle the "housing crisis", a charity says.
The Intergenerational Foundation calls for tax breaks to encourage downsizing and help free up some of the estimated 25 million unused bedrooms in England.
More than half of over-65s are in homes with two or more spare bedrooms, which could be used by families, says its report based on government figures.
Ministers said they did not believe in "bullying" people out of their homes.
The foundation (IF) is a new group set up to campaign on financial issues, particularly those affecting younger people, such as affordable housing and job prospects.
It says that while many people are living longer and staying in what was once their family home, younger families are being squeezed into smaller properties.
"The 'housing crisis' is increasingly an issue of how our housing stock is shared between younger and older generations," said IF co-founder Angus Hanton.
"The divide between the housing 'haves' and 'have nots' has moved from being one dominated by wealth or class to one dominated by age."
The report, entitled Hoarding of Housing, said that 37% of homes in England - about eight million - were under-occupied - meaning they had at least two unused bedrooms. This is up from 20% four decades ago.
After steady growth in home ownership since World War II in England, it has been in decline since 2003.
This is because ownership among under-35s is falling faster than it is rising among over-65s, IF said.
"It is perfectly understandable that retired people cling to their home long after it has outlived its usefulness as a place to bring up a family in," said report co-author Matthew Griffiths.
"But there are profound social consequences of their actions which are now causing real problems in a country where new house-building is almost non-existent."
IF suggested encouraging older people to downsize by exempting over-60s from stamp duty when they sold to move to a smaller home.
The campaign group also urged the government to consider replacing council tax with "a proper land tax, to reflect the social cost of occupying housing, particularly housing that is larger than one's needs".
Housing Minister Grant Shapps said: "Whilst this report makes interesting reading, we do not agree that people should be taxed or bullied out of their homes.
"Instead we will work with families to ensure that housing becomes more affordable over time."
He pointed out that £13m was being provided to councils to make it easier for tenants to move from larger to smaller homes.
The FirstBuy scheme had been introduced to help people get on the property ladder, Mr Shapps added, and house building programmes were going to deliver 370,000 new homes.