Google has apparently pulled a number of apps from the Play Store after it was warned they featured intrusive advertising, although millions of downloads had taken place before it took its action.

According to mobile security firm Avast, which investigated the issue, some of the apps even waited up to 30 days before showing their “true colours”, by which point it may not be clear which downloads had caused the abnormal behaviour.

The apps presented users with warnings about a problem, for example a security breach, which was not true. These led 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”.

Avast also said that some users were directed to legitimate security apps on Google Play, leading the vendor to question if the vendors actually wanted to promote their products via adware.

ZDNet subsequently reported that Google had taken action, but one of the apps, card game Durak, was already in the 5-10 million download category. Other affected apps were an IQ test and a history app.

While Google has been making efforts to prevent malicious apps getting into the Play Store, the size of its catalogue means this is likely to remain an uphill challenge.

And with advertising obviously being a legitimate way to monetise apps, it is obviously a challenge to enable legitimate use while preventing aggressive implementations – especially when these may not be apparent for some time after installation.