BlackBerry is reported to be beginning user tests of its cross-platform BlackBerry Messenger app, ahead of its full release later this year.

According to blog BlackBerryOS, selected users are being invited to test the app, which is available for Android 4.x and iOS6 platforms. It is not clear what the criteria for participating in the pilot are.

BlackBerry Messenger has been a core feature of the BlackBerry platform for some time, and has been instrumental in building and maintaining the ailing smartphone maker’s user base in many markets.

But after months of speculation, the company announced it would open BBM to users of other smartphones earlier this year.

The opening of BlackBerry Messenger to other platforms creates numerous issues for BlackBerry.

Firstly, with the app previously being a central (and exclusive) part of the BlackBerry user experience, dedicated BBM users (particularly among young users and in emerging markets) may now opt for alternative smartphones – primarily low-cost Android devices – impacting the vendor’s core devices business.

Secondly, while BlackBerry may now be able to build a large, cross-platform user base of BBM, this will primarily be for free communications services. It will face similar challenges in monetising this to a number of other “over-the-top” messaging app providers, which similarly offer their core services free-of-charge.

And finally, there are of course numerous alternatives, many of which will have already won-over BlackBerry device users – such as WhatsApp, Nimbuzz, KakaoTalk, Viber and Line.

BlackBerry previously promised availability of BBM for Android and iOS “this summer”.