Android users are being warned that several popular apps that were on the official Google Play store appear to have contained hidden code that made malicious ads pop up.
Security firm Avast said that one of the apps involved - a free version of the card game Durak - had been downloaded up to 10 million times, according to Google Play's own counter.
Google has now blocked access.
But one expert noted that the problem might be less widespread than feared.
Avast said that it first became aware of the issue after a member of the public contacted it after carrying out his own investigation into how his Nexus 5 smartphone had come to be infected with malicious code.
The "adware" was causing spurious pop-up messages to appear that had been made to look like system notifications. These told him his phone was running "slow" and that he needed to install new software to fix the problem.