Some UK letting and estate agents are failing to notify the Information Commissioner's Office (ICO) that they are handling people's personal information.
But it is a legal requirement under the Data Protection Act for all organisations handling personal information to notify the ICO, the privacy watchdog said.
As property agents routinely process personal data, most would be subject to the Data Protection Act and would need to register as data controllers urgently, the ICO said.
Only 3,734 estate agents and 1,416 lettings agents appear on the public register, which represents a small proportion of the industry.
Mick Gorrill, head of enforcement at the ICO, said, "We want to work with the industry to ensure all property agents meet the legal requirement."
The ICO is to contact industry bodies to encourage members to register, but will take action against those who flout the law, he said.
"The message is very clear: notify with the ICO or face regulatory action," said Gorrill.
In the past year, the ICO has prosecuted seven organisations and individuals for failing to notify the privacy watchdog that they are processing personal information.