The 25-year-old picked up the injury in his club side's 15-15 draw with Ospreys in the European Champions Cup on Saturday evening, forcing him to make way midway through the match.
Vunipola will see a specialist this week to determine a time frame for his return but he is nearly certainly out of England's Six Nations opener on 4 February in Italy and his participation at any stage of the tournament is now in serious doubt. A subsequent x-ray has confirmed the worst and while the recovery time for a fractured forearm can vary greatly, Ireland's Joey Carbery suffered a similar injury in mid-November and is yet to return.
England could be sweating on the fitness of Billy Vunipola after he sustained an injury playing for Saracens this weekend.
England are already likely to be without Nathan Hughes for the entire Six Nations because of a knee injury, while back-row option James Haskell could be in disciplinary trouble over a high tackle. He was however, sanguine following his latest setback, saying on social media, "One of these days I might actually be able to play more than two games without breaking (no pun intended)".