Average home prices in the capital slid 0.6 per cent in the third quarter or July-September period compared with a year earlier, home-loans provider Nationwide reported in its latest survey.
That was the first year-on-year decline in the index for London since the third quarter of 2009.
However, Nationwide also said that average prices across Britain rose 2.2 per cent over the same period from a year earlier.
"Low mortgage rates and healthy rates of employment growth are providing some support for demand, but this is being partly offset by pressure on household incomes, which appear to be weighing on confidence," the lender said in a statement.
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British households' purchasing power has taken a hit because of the rising cost of imports after last year's referendum vote to leave the European Union, which pushed down the value of the pound.
British salaries fell by 0.4 per cent between May and July compared to the same period a year earlier when adjusted for inflation, recent official data showed.
Britain's property market could also suffer if the Bank of England were to raise its main rate from a record-low of 0.25 percent, which would push up the cost of mortgages.