There were 20% fewer transactions across England and Wales last month compared with September last year.
LSL and Acadata put the total number of transactions in September in England and Wales at 63,000, down a fifth on the same month last year.
Annual house price growth also slowed, LSL and Acadata jointly reported this morning.
The average house price was £297,287, down just 0.1% on the previous month with annual house price inflation standing at 1.3%.
However, the slowdown was almost entirely led by London the south-east.
Stripping out these regions, annual house price growth was 3.3%, while in Wales the market is strengthening with transactions holding steady.
Oliver Blake, managing director of LSL brands Your Move and Reeds Rains, said: “Despite slowing price growth, particularly in the southern regions, the north continues to report positive results.
“The future, however, will rely heavily on stock availability, and with housing clearly on the political agenda, what government support may be offered to those looking to buy.”
Regions with house price growth included the north-west but not the north-east.
While Greater London, the south-east and the east of England all showed average prices falling away, the south-west had average house prices rising.