Charles Leclerc's Monaco jinx struck again on Sunday as Ferrari's pole-sitter retired from his home Formula One race before the start due to a problem with his car.
His absence from the grid left Red Bull's title contender Max Verstappen alone on the front row with Mercedes' Valtteri Bottas in the third slot but effectively second.
Mercedes' championship leader Lewis Hamilton, who is 14 points clear of Verstappen after four rounds, moved up to sixth on the grid from seventh.
Ferrari said in a statement it had detected a left driveshaft problem that was impossible to fix in time for the start.
The 23-year-old had qualified on pole position in the principality after crashing in the final seconds of the session, bringing out red flags that prevented any of his rivals from doing better.
The immediate concern then was for the car's gearbox, with a replacement triggering an automatic grid penalty, but that was given a clean bill of health by the Italian team on Saturday and again on Sunday.
The driveshaft problem was unexpected, raising questions of how Ferrari had failed to detect it earlier and whether they had taken a gamble to protect pole.
"I am surprised because they checked the gearbox," said Mercedes boss Toto Wolff.
"It's also a shame because he's Monegasque, people are cheering for him and he deserved it (pole) because his lap was spectacular.
"For us, Max mustn't score 25 points. That's not good for us. So we would have hoped that Charles stayed in the lead but let's see what opportunity it gives for Valtteri."
Leclerc has yet to finish a home grand prix, with the showcase harbourside race not held last year due to the COVID-19 pandemic.
He retired after collisions in 2018 and 2019. The Monegasque also failed to finish two Formula Two races there in 2017.
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