Jenson Button is set to start last for what could be his final Formula One race after being hit with a 15-place grid penalty at the Monaco Grand Prix .
Button, 37, is is making a one-off return to Formula 1 this weekend following Fernando Alonso's decision to participate at the Indianapolis 500.
Following a series of changes to the parts on Button's Honda engine after an issue was detected following second practice at Monaco on Thursday, Button is set to start his one-off race from the back of the grid.
Honda announced on Saturday morning that a problem had been discovered in the MGU-H on Button's car following Thursday's practise sessions and that it required both the MGU-H and turbocharger to be replaced.
As both were the fifth such components fitted to the car this season, that exceeded the four-per-year limit and prompted the penalty.
Although Monaco is Button's first appearance of 2017, such penalties are decided by use-per-car so Alonso's problems earlier in the season were included.
Honda also changed the internal combustion engine, MGU-K, MGU-H, and turbocharger in Stoffel Vandoorne's car before Thursday and Saturday, but those change "were scheduled in order to control the mileage".
Only the MGU-K was new, and is Vandoorne's third of the year, with the other parts re-used from previous races, so Vandoorne does not receive any penalties.
McLaren's season has been derailed by a series of problems with their engine supplier Honda. The British team prop up the constructors' championship having failed to score a single point this year.
Button was 12th-fastest in practice on Thursday ahead of the 306th race of his Grand Prix career.
The 2009 world champion had described his return in Monaco as a "dream", and the latest reliability failure is the last thing he would have wanted having seamlessly adjusted back to life in F1.
"It's awful for him and for McLaren as well," McLaren boss Eric Boullier said. "It was not expected at all."
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