Research collected by the association has found that private rents have fallen every year in real terms since 2016, now sitting nearly 10% lower when compared to the Retail Price Index.
Figures from the Office for National Statistics (ONS) show that rents in the capital have fallen by 9.6% between April 2016 and February 2021.
Compared to the Consumer Price Index, including housing costs, rents fell by 5.1% over the same period.
The NRLA has warned that any move to control rent rises by linking them to inflation would therefore risk leaving tenants in a worse position.
In his manifesto for re-election as Mayor, Khan has called for the power to introduce rent controls in London.
The NRLA also pointed to a report published by the Treasury in 2010, that outlined the negative impact such a policy could have on inner city housing.
Assessing the impact of rent controls before they were abolished in 1988, the report concluded that they had been a major factor in the “decay of much of the inner city housing stock.”
Ben Beadle, chief executive of the National Residential Landlords Association, said: “Rent controls would be a disaster for London as the last Labour Government made crystal clear.
“They would mean tenants actually paying higher rents than leaving them to market forces.
“The story of rent controls wherever they have been introduced is that they exacerbate an already serious shortage of available homes.
“Rather than calling for things he cannot deliver, the Mayor should focus on using the powers he already has to boost the supply of available housing, including for private rent.”
