Analysts JP Morgan estimate that Korean firm Samsung has scored a major hit with its new phone, reports French newspaper Les Echos (via Into Mobile).
The analysts believe that Samsung sold 6.5m Galaxy S3s worldwide between its May 29 release date and the end of June.
JP Morgan also predicts that this will increase to 10m in July and then double to 20m by the end of the third quarter (July to September), taking just six months to achieve the sales feat.
It is thought that the Galaxy S3 has been boosted by its simultaneous launch in both the US and Europe.
Predecessor the Galaxy S2 took five month to sell 10 million units, but that was largely because American customers did not receive the phone until six months after their European counterparts.
The Galaxy S3 is thought to be outselling its main rival, the HTC One X, but its toughest test will come later in the year, when Apple is expected to launch the new iPhone.