The RICS house price balance - measuring the difference between the percentage of surveyors reporting price rises and those seeing a fall - fell sharply to +32 in September from +51 in August, signalling a slowdown in price growth.
"Looking further out, the picture portrayed by the RICS survey has clearly shifted in a negative direction," he added.
After booming during and after the COVID-19 lockdowns as home-owners sought bigger properties, Britain's housing market has cooled recently.
Rival lender Nationwide says British house prices failed to rise in monthly terms for the first time since July 2021 in September.
A recent cut to Britain's stamp duty tax on property purchases, part of finance minister Kwasi Kwarteng's package of tax cuts, was set to be outweighed by the rise in mortgage costs, RICS said.