The data is based on Hamptons analysis of activity among Countrywide agents and the homes they sell.
A higher proportion of homes are now sold discreetly in prime country markets (24%) than in London, Hamptons said, while 59% of off-market sales are now outside the capital.
With 10% of homes sold off-market nationally, the absolute numbers are running above 2015 levels, the agent said.
The growth in off-market transactions has increasingly been driven by lower-priced properties, the research suggests.
So far this year, the average discreetly marketed home changed hands for £858,000, down from £979,000 in 2021.
Nationally, around one in ten homes sold this year found a buyer without being publicly marketed, the highest level since 2015.
Additionally, the average off-market home achieved 99.5% of its asking price, while similar homes marketed more widely achieved an average 99.1% of their initial asking price.
These record prices are being achieved off the back of increasingly shorter marketing periods.
The average home sold away from the glare of the open market took an average of 42 days to find a buyer, compared with 65 days for a similar prime home that didn’t start life off-market, Hamptons said.
Meanwhile, the average time to sell across the whole of the market in May stood at 26 days, up from 24 days in May 2021.
With more off-market properties securing a sale quickly, fewer homes are then being advertised more widely later down the road, Hamptons said.
Just over a quarter (26%) of homes marketed discreetly came onto the open market this year, down from 38% pre-pandemic in 2019.
Aneisha Beveridge, head of research at Hamptons, said: “Selling off-market has become an increasingly established sales strategy over the last five years.
“The first generation of off-market sellers were those people primarily concerned about the privacy of their home, keen to ensure it wasn’t exposed to anyone who wasn’t serious about buying it.
“While these sellers still make up around half of homes launched off-market, they were joined in 2016 by central London homeowners keen to minimise their digital footprint in what was an increasingly tough market.
“Post-pandemic, selling off-market has increasingly been driven by sellers keen to avoid wider marketing and limiting the number of buyers through their doors.
“This strategy has paid off. Buyers have been willing to pay a premium to secure their home off-market and prevent sellers from marketing the property openly to other interested parties where competition is rife.”
Beveridge said it is likely that off-market sales have reached a peak.
She added: “With the number of homes on the market forecast to rise later in the year, buyers are likely to be more cautious about paying a premium in the face of an increasing amount of choice. If this happens, off-market sales may retreat back into their prime heartlands.”