Foxtons says revenue from property sales plunged 23% last year due to uncertainty following the Brexit vote as well as changes to stamp duty.
Profits at the London-focused estate agent more than halved as a result, falling to £18.8m last year from £41m in 2015.
Foxtons does not expect any let-up in 2017, warning trading conditions will "remain challenging".
Sales will fall further this year if current trends continued, it said.
"Last year's London property market was severely impacted by an unprecedented sequence of events with changes to stamp duty and the EU referendum vote leading to a substantial reduction in property sales transactions, especially in central London," said chief executive Nic Budden.