Google's Gmail Android app has started showing adverts which cannot be turned off without disabling the 'Promotions' folder.
The adverts initially appear along the lines of emails within the 'Promotions' inbox of the Gmail app, revealed a Dutch Android blog, Android World.
But the 'Promotions' inbox can be switched off within Gmail's inbox settings on both the desktop and Android versions of Gmail.
However, many Gmail users view the folder to look at offers they are interested in.
Reports also reveal that the latest ad units are marked as ads by being displayed in different colours than conventional emails.
It is not clear whether ads will be limited to the 'Promotions' inbox in the long term, or if they could feature in other inboxes within the Gmail app.
A report from ABI Research suggested that Android mobile app revenue would double to about $6.8bn by the end of 2013.
And in a year's time, Android smartphone app revenue is expected to rise from 59.1% to 65.9%, with the smartphone app market to generate most of Android's revenue.
Advertising has been a major contributor to Android's app revenues, which is based on Google's analytics search engine.
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