Keating had put the sole Chevrolet Corvette C8.R on pole by 1.5 seconds during the Hyperpole session on Thursday and the car he shares with Nicky Catsburg and Nico Varrone was among the favourites to take the class spoils - and it currently leads the GTE Am World Endurance Championship standings.
However, in the night-time session following the Hyperpole contest, Keating was deemed by stewards to have failed to slow sufficiently and will therefore have to serve the stop/go penalty during the opening four laps of Saturday's race.
The penalty is a blow for Corvette's hopes of ending its eight-year win drought at Le Mans in the final year of the GTE division featuring at La Sarthe.
It was not Keating's only penalty of the session as he was also fined for speeding in the pitlane.
Prior to qualifying, Keating had spoken of how he was trying to repeat his success in the class last year, when he won as part of TF Sport's Aston Martin line-up.
Source: Autosport