The average 21 – 29 year old spent 34% of their pre-tax income on renting a room in 2019, according to the Hamptons Lettings Index.
This figure is down from 39% recorded in 2017 thanks to stronger income growth.
It reportedly costs 31% more to rent a one-bedroom home than it does to rent a room in a shared house.
Brighton is the least affordable city to rent a room, closely followed by London.
The average rent of a newly let property rose to £998 pcm in August, 2.3% higher than the same time last year.
Portsmouth recorded the strongest room rental growth with the average cost of a room rising year-on-year by 7% to £467.
Aneisha Beveridge, head of research at Hamptons International, said: “Tenants in their twenties spend a third of their pre-tax income on room rents in Great Britain.
“Yet the cost of trading up to rent a one-bed would take up nearly half of their earnings.
“With its large student population putting pressure on rental accommodation, Brighton is the most unaffordable city to rent a room in Great Britain with London following in second place.
“Rental growth on newly let properties has doubled since August 2018. Rents rose 2.3% in August, more than twice the 1.0% recorded in August 2018.
“But this isn’t even across the whole of Great Britain. The South East and South West recorded the strongest rental growth, meanwhile Scotland, Wales and the Midlands posted rental falls.”