Jenson Button will start the Monaco GP from the pitlane after his 15-place grid penalty due to changes to his Honda power unit.
Although Button was ninth in Saturday's qualifying shootout, the former world champion, making a one-off return to Formula 1 by standing in for Fernando Alonso, will have to line up last due to another reliability failure for Honda.
The MGU-H and turbocharger on Button's car had to be changed after McLaren's engine makers detected an issue following Thursday's second practice session.
Both the new MGH-H and turbocharger are the fifth elements of the year to be used on that car, with Button having inherited Alonso's four-per-car limit for 2017.
With the team making subsequent changes to his car out of parc ferme conditions on Saturday night, Button will now start from the pitlane rather than at the back of the grid.
"The penalty still hurts, it doesn't matter how many races you've done," said Button after qualifying. "They told me yesterday and I was like 'Ohhh'. It's a shame but it's never easy."
"It's awful for him and for McLaren as well," McLaren boss Eric Boullier added. "It was not expected at all."
However, Button has insisted he has relished his return to F1.
"I've enjoyed the weekend a lot," he told Sky F1. "It's my last qualifying session but probably my most enjoyable."
While Alonso impressively qualified seventh in Spain, his last race before his Indianapolis 500 attempt, it has been a nightmare campaign for McLaren-Honda, the only team without a 2017 championship point.
And though the MCL32 looked more comfortable in practice around a narrow street circuit which somewhat negates Honda's lack of power, the latest penalty means points are highly unlikely this weekend.
Stoffel Vandoorne is already serving a three-place grid penalty thanks to a collision with Felipe Massa in Barcelona.
As for Button, the 2009 world champion who described his return in Monaco as a "dream", the latest reliability failure is the last thing he would have wanted having seamlessly adjusted back to life in F1.
"It's soul-destroying for Button," said Sky F1's Paul di Resta. "When is there going to be light at the end of this tunnel?"
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