Hamilton attempted to overtake his former McLaren teammate into the Les Combes right-hander on the opening lap.
The Mercedes driver misjudged it and failed to leave Alonso adequate room at the apex, causing the two to collide.
The contact sent Hamilton’s Mercedes airborne before banging back onto the track in violent fashion.
Hamilton tried to limp back to the pit lane but the damage was too severe, forcing him to stop out on track on the run to Blanchimont.
The 37-year-old took full responsibility for the incident afterwards but incurred a warning from the FIA for failing to visit the medical centre following the incident.
Speaking after the race, Hamilton admitted he was fortunate to come away without any serious injuries.
“Well, yeah, I almost broke my back coming down,” Hamilton said. “So it’s a big, big hit. I remember just looking at the ground.
“So it was quite high. It was definitely high up. I’m grateful to be still alive and in shape.”
Hamilton had hoped he would have been to continue but the car had sustained too much damage.
“I could hear something’s broken in the gearbox,” Hamilton added. “So yeah, I mean, coming down I would have broken so much in the back end.
“So I was told to stop. But obviously in that moment, you’re hopeful you can keep going. But yeah, not meant to be.”
Hamilton sits in sixth-place in the F1 drivers’ championship and is on course for his first-ever winless campaign.