Lewis Hamilton said he ruined his chance of winning the Azerbaijan Grand Prix by mistakenly hitting the wrong button on his steering wheel.
At the restart which followed Max Verstappen's crash out of the lead, Hamilton made a better start from second position than Sergio Perez on pole position. However, the seven-time world champion locked up his brakes and went straight on at Turn 1, dropping him to 16th.
Hamilton later asked Mercedes if he had accidentally turned on the team's "magic" brake button, which helps drivers better warm their tyres during formation laps and in the build-up to Safety Car restarts when the pace is much slower than a normal race.
The button is usually switched off for the race start so the brake balance can return to an ideal setting for laps at racing speed.
On this occasion, Hamilton inadvertently hit the switch after the lights went out at the restart when reacting to Perez moving across at the restart.
"On that restart I think when Checo moved over towards me I clipped a switch," Hamilton said. "It basically switches the brakes off, so I just went straight. I had no idea that I'd even touched it.
"It's very hard to take. But I'm mostly just really sorry to the men and women in the team who've worked so hard for these points. We will regroup and come back stronger."
With title rival Max Verstappen crashing out after a late tyre failure, Hamilton had looked set to reclaim the lead of the drivers' championship.
He added: "Naturally, it's quite a humbling experience to be honest. We'd worked so hard this weekend to come back from out of the top 10. It was looking so good.I'd put everything on the line, I'd fought as hard as I could today."
With both Verstappen and Hamilton failing to finish, the Dutchman remains in the lead of the championship by four points.
Hamilton admitted there was an element of fortune to the way the race had unfolded in terms of Verstappen not finishing.
"Very unfortunate for Max obviously. I would say today is a good stroke of bad luck. It is what it is and we'll try and regroup and come back stronger in the next race."