Landlords face fines of up to £4,000 if properties rented out from April 1 are not rated EPC E or higher, according to ARLA Propertymark.
There are still around 300,000 rental homes that are rated F or G, although this number has fallen from 700,000 in 2012, the trade body estimates.
Landlords have now been warned that time is running out to make sure their properties comply with the minimum energy efficiency standards.
It is also feared that landlords who don’t comply with the new standards could run into problems when trying to remortgage.
David Cox, chief executive of trade body ARLA Propertymark, said: “Landlords now only have one month to ensure their properties comply with the Government’s minimum energy efficiency standards.
“Landlords have been responding to tenants’ demands for better quality and better insulated homes over the last few years.
“However, there are still a lot which don’t meet the standards.
“After the deadline passes at the end of this month, landlords face fines of up to £4,000 for flouting the law, or losing money on empty properties which can’t be let until they meet the standards.
“This will also have implications on the wider rental market, which is in the midst of a serious supply crisis.
“There are Green Deal finance plans available and you can contact companies such as Propertymark’s industry supplier ECO-Energi to find out how to upgrade your property for a low cost.
“For any landlords who haven’t yet started to upgrade their properties, now is the time to act – and quickly.”