The report states that a quarter of 20 to 34-year-olds live at home and since 1998 this has risen by 41% in London, where housing is most expensive, but by much less in cheaper areas like north-east England (14%) and Yorkshire and the Humber (17%).
And for 23-year-olds across the UK, the proportion living with parents has risen from 37% in 1998 to 49% in 2017.
He added that it was essential the government took this into account when forecasting future housing need.
The study also suggests youngsters who do move out are much less likely to live on their own than they were in the late 1990s and single-person households have dropped to 30% in recent years.
This is in stark contrast to most of northern and western Europe, the report says, where single living has been increasing rapidly.
In France and the Netherlands, 35% of households are single-person. And this rises to more than 40% in Germany and Denmark.
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