The competition watchdog has blocked Carsales.com's takeover of the Trading Post website as it would reduce competition in the car classifieds market.
Carsales had planned to buy TradingPost.com.au from Telstra and keep it running using staff and infrastructure from its quicksales.com.au website.
The Australian Competition and Consumer Commission (ACCC) said the proposed deal would reduce the choice for advertisers by removing a significant competitor to Carsales from the online advertising market.
"The ACCC concluded that the proposed acquisition is likely to result in a substantial lessening of competition through the removal of a close and effective competitor of Carsales," ACCC chairman Rod Sims said in a statement.
"The proposed acquisition would significantly increase Carsales market power and competition would be substantially reduced to the detriment of automotive dealerships and private advertisers."