Around 13.7 million bedrooms in owner-occupied households across the country are currently lying "idle", according to flatsharing website easyroommate.co.uk.
Based on current average flatshare rents, these rooms could potentially generate £5.8 billion a month or around £69 billion a year if they were opened up to renters, the website estimated.
London home owners were found to be missing out on the biggest pool of potential rents. The capital was found to have more than four million unoccupied bedrooms which could result in rents of over £28 billion a year if they were let out at the average monthly flatsharing rent in London of £580.
Easyroommate based its estimates on a combination of English Housing Survey figures, its own research and average rents recorded by lettings network LSL Property Services, owner of Your Move and Reeds Rains.
Flatshare rents have risen by 4% over the last year to reach £390 a month typically.
Based on average rents, home owners in London, the South East, East Anglia and the South West would go over the threshold if they rented out a room for a year, the study said.
Jonathan Moore, director of easyroommate.co.uk, said: "It's been 16 years since the current rent-a-room relief threshold was implemented. Increasing the threshold would tempt more owners into the market to provide much-needed accommodation to the private rental sector.
"Not only would this help ease the burden being placed on rental property, but the savings made by those renting in flatshare or lodger accommodation can help build the deposit needed to climb on the sales ladder."