Sales of super-prime properties in London - luxury pads costing over £10m - slumped this summer.
According to research by property consultancy London Central Portfolio (LCP), sales have fallen 86 per cent in the three months to August as compared to the same period last year.
The drop in activity could mean a £45m hit to stamp duty receipts, LCP said.
The UK's richest property clients shunned new build properties; no new builds worth more than £10m were sold in the three month period. During this period last year, new builds accounted for 23 per cent of super-prime sales.
The news comes after Knight Frank reported that house prices on high-end homes in central London have been falling. In Chelsea, prices have dropped by as much as 10 per cent since last year.
Naomi Heaton, chief executive of LCP, said: "Overall, this slowdown in the luxury property market - a big contributor for the exchequer and UK economy - is very concerning.
"As the government faces the daunting task of negotiating Brexit together with a potential slowdown in the UK economy, it should consider its strategy on residential property taxation carefully to ensure it meets its objectives of increasing revenues."