The public are being asked to inform the authorities if they know of agents operating fee-fixing cartels.
The request - from the Competition and Markets Authority - follows the news that three Berkshire estate agents have been fined more than £600,000 for illegally fixing the minimum commission rates they charged their customers.
Now the CMA says anyone who has information about any other cartel is encouraged to call the so-called cartels hotline on 020 3738 6888 or email email@example.com.
This is only the latest in a series of penalties levied on agents by the CMA..
Others include fining three members of the Three Counties Estate Agent Association £735,000 for breaking competition law in relation to letting and estate agent fees, and fining four estate agents in Somerset over £370,000 for colluding to set minimum commission rates.
The CMA runs a Stop Cartels campaign which aims to educate businesses about which practices are illegal and urges people to come forward if they suspect a business has taken part in cartel behaviour, such as rigging contracts or price fixing.
In the CMA’s latest ruling on estate agents, the companies Michael Hardy, Prospect, Richard Worth, and a fourth company, Romans, broke competition law by running a cartel which set minimum commission rates for properties in Wokingham, Winnersh, Crowthorne, Bracknell and Warfield.
Romans escapes financial penalty because it informed the CMA about the cartel.
All four agencies regularly discussed private pricing details after initially fixing fees in September 2008.
“It is disappointing we've found yet another case of estate agents breaking competition law. We trust that the fines issued today will reinforce our message that we expect the sector to clean up its act and make sure customers are not being ripped off in this way” says a CMA spokesperson.
"The industry needs to take note - this kind of behaviour will not be tolerated. If you break the law, you risk similar consequences."
Prospect has been fined £268,765, Michael Hardy will have to pay £142,843 and Richard Worth - now run by a different firm - has been fined £193,911.
Prospect says it has “been able to learn from past mistakes from years ago" while
Michael Hardy's managing director, Neal Mackenzie, insists the firm adopts “robust procedures” to ensure no repeat could occur.
Romans' chief executive Peter Kavanagh says a small number of staff "had acted in a manner totally contrary to the standards and values of the company".