The incident occurred on the penultimate lap of the 44-lap contest as Magnussen, trying to defend fifth place from Alonso on the approach to Les Combes, edged the Spaniard onto the grass.
The stewards immediately announced an investigation and, having heard from both drivers and examined video evidence, determined that Magnussen was at fault.
“The driver of car 20 [Magnussen] was defending his position on the straight between turns 4 and 5,” read the official stewards report. “A significant portion of car 14 [Alonso] was alongside car 20. The driver of car 20 did not leave enough space for car 14 and forced the car off the track.”
The penalty drops Magnussen from sixth to 12th place, denying the Dane his best finish since the season-opener in Australia and lifting Force India’s Nico Hulkenberg into the final points-scoring position of tenth.