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.

If he followed the on-screen prompts he was then directed to download other apps, only some of which were legitimate.

"You get re-directed 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," wrote Avast's malware analyst Filip Chytry.

The original apps were said to have held off showing the pop-ups until seven or more days after being installed, which the analyst suggested was intended to mask the fact that they were responsible.

"Most people won't be able to find the source of the problem and will face fake ads each time they unlock their device," he wrote.

"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."

In addition to the Durak card game, other apps alleged to be involved include:

A Russian language IQ test, which Google Play indicates had been downloaded up to five million times

A Russian history educational tool, which Google Play indicates had been downloaded up to 50,000 times

A search by the BBC for other apps made by the same developers revealed dozens more apps, including video games, a psychology guide, wedding planning software and cookery tips - all of which have now been blocked.

(BBC)