Google is facing increasing concerns about adware infecting its Android operating system, which is used on smartphones and tablets, the BBC reported.

The attention comes after several popular applications, which were available for download on the official Google Play store, turned out to contain hidden code that made malicious ads pop up, the channel added.

One of the applications in question -- a free version of the card game Durak -- had been downloaded up to 10 million times, security firm Avast said based on Google Play's own counter. The company said it first became aware of the issue because of a user who had carried out his own investigation into how his Nexus 5 smartphone had become infected.

The malicious software creates pop-up messages that look like system notifications and warn that the phone is running "slow." They suggest users download new software to fix the phony problem, directing them to other apps - only some of which were legitimate.

Filip Chytry, a malware analyst for Avast, said that most smartphone owners lack the technical expertise to find the source of the pop-ups and end up confronted by fake ads every time they unlock their device.

"You get redirected to harmful threats on fake pages, like dubious app stores and apps that attempt to send premium SMS behind your back or to apps that simply collect too much of your data for comfort while offering you no additional value," Chytry wrote.

The malware seems to have been cleverly designed, as its code includes a 30-day waiting period before it activates its features, Information Week noted.

"I believe that most people will trust that there is a problem that can be solved with one of the app's advertised 'solutions' and will follow the recommended steps, which may lead to an investment into unwanted apps from untrusted sources, Chytry added.

That adware-toting applications on the official Google Play store was surprising, the Guardian judged, because users generally encounter malware issues when they download such programs through unofficial providers. Nevertheless, the software giant has finally taken swift action to remove the apps in question.