Lewis Hamilton says flicking a "magic" button on his Mercedes car caused him to blow his chance of an Azerbaijan GP victory, with the world champion inadvertently "switching the brakes off" and missing the first corner just as it appeared he had passed Sergio Perez at the restart.
Hamilton looked to have lucked in at Baku after championship and race leader Max Verstappen crashed out, and - second on the grid for the post-red flag restart - he then got the jump on Perez off the line.
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But, in dramatic scenes on an extraordinary afternoon, Hamilton then locked up into Turn One and ran onto the escape road, dropping him way down the order and out of the points.
On team radio, a devastated Hamilton asked his Mercedes team: "Did I leave the magic on? I could have sworn I turned that off."
He then told Sky F1 after the race that he mistakenly pressed the 'magic' button just when it appeared a victory was within his reach.
"On the restart, I think when Checo [Perez] moved over towards me I clipped a switch and it basically switches the brakes off so I just went straight," said Hamilton. "I had no idea that I'd even touched it."
The 'magic' switch is essentially designed to heat the front tyres on the car before a race start by changing the brake balance. Hamilton likely turned it off before the start before hitting it again by accident.
"He touched a button and the brake balance changed," explained Mercedes boss Toto Wolff. "The brake balance went forward and basically the car doesn't stop."
The mistake means Verstappen retains his four-point championship lead, while Red Bull have stretched their advantage over Mercedes in the constructors' standings as Valtteri Bottas also finished outside the top 10.
Hamilton offers apology to Merc | 'Very hard to take'
Hamilton said he was "very sorry" to his Mercedes team for the error, having got them in contention despite Red Bull appearing to have a faster package around the Baku street circuit.
"Naturally, it's quite a humbling experience to be honest," added Hamilton to Sky F1's Rachel Brookes.
"We've worked so hard this weekend to come back out of the top 10 and it was looking so good. I put everything on the line, I fought as hard as I could today.
"It's very hard to take. Mostly I'm just very sorry to the men and women on the team, who have worked so hard for these points. We will rebuild and come back stronger for sure."
Asked about Verstappen, who crashed out due to a tyre puncture with just five laps remaining when leading the race, Hamilton said it was "very unfortunate for Max, obviously".
"Today's a good stroke of bad luck," he admitted. "It is what it is. We'll just try and regroup and come back stronger on the next race."