More than a third of homes in England are "under-occupied," putting pressure on younger generations as older people remain in their properties for longer, a study has suggested.
The Intergenerational Foundation, which released the report, called for tax breaks to encourage older people to downsize.
Its Hoarding on Housing report said that 25 million bedrooms are empty in England's homes.
The document, based on English Housing Survey figures, found that 51.5% of over-65s live in homes with two or more bedrooms that they do not need, and half of single households where the owner is aged over 60 have three spare bedrooms or more.
The document said that more than a third of homes are "under-occupied," up from a fifth in 1971. These are classed as households with at least two bedrooms more than they require.
"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 the report's 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."
The document said that in 2008/09 three-quarters of over-65s owned their own home, up from 49% in 1981. It said the age group with the largest levels of home ownership is 55 to 65, with an 80% home ownership rate.
The report said: "The ageing UK population will be one where outright home ownership will be the norm. It will also be one dominated by small households."
Unemployment and debt meant "home ownership is being replaced by the private rented sector as the likely default housing option for younger people". More than 70% of those in the private rented sector are aged between 16 and 44, the study found.
The Intergenerational Foundation said it was not urging the Government to "turf out" older generations. It called for reforms such as exemption from stamp duty for the over-60s when they move to a smaller property, and overhauling the council tax system.