According to WABetaInfo, a new update was hidden in code for Messenger which would enable users to speak to friends on WhatsApp without downloading the latter app.
However, it's not known when the merge would take place and it could take some time, as it's a "complicated" process.
The tech leaker said: "The integration of WhatsApp into Facebook Messenger isn't available yet.
"This is a very complicated feature and it requires time. We've no idea if the plan to merge those services will continue or if it will be abandoned."
In April, Facebook launched a new cross-platform app called Messenger Rooms, which allows users on a range of apps, including Facebook Messenger, WhatsApp and Instagram to speak together in one place.
Whilst there was also talk last year of Facebook and its sister apps WhatsApp and Instagram joining together for a single messaging service.
Meanwhile, WhatsApp recently enabled users to have video conferences with up to eight people.
The messenger app rolled out the new update amid the coronavirus pandemic, which has seen people hosting virtual parties and calls with multiple family members on rival apps such as Zoom, Houseparty and FaceTime.
Android and iPhone users can now speak with twice as many people now the limit has been expanded.