More than 1,000 homes lying empty over a long period in a London borough are owned by private landlords.
The GMB union is now calling for landlords of empty properties to be hit with tough penalties.
Altogether 3,638 homes in Croydon are empty, and the number of long-term empty properties owned by private landlords is 1,321.
GMB says that 2.4% of Croydon’s homes are currently empty.
Despite the numbers, the council is one of several authorities looking to send homeless people outside London because of a shortage of affordable accommodation.
Over London as a whole, there are 74,553 empty dwellings.
GMB says there should be strong penalties against landlords who leave their properties empty.
Paul Hayes, GMB regional secretary, said: “The system needs to be tilted in such a way that property owners face penalties if they do not take all reasonable steps to ensure that dwellings are occupied.”
A spokesperson for Croydon Council said the authority wanted to work with private landlords on bringing their properties back into use, but would take enforcement action and issue compulsory purchase orders in cases where owners refuse to co-operate.