Jenson Button fears that McLaren may not score points all season if it does not get to the bottom of a 'scary' Spanish Grand Prix.
The Briton was left struggling with an unexplained car problem that meant he was suffering wild wheelspin whenever he touched the throttle.
Speaking about it afterwards, Button said: "The first 30 laps were not boring – they were the scariest 30 laps of my life.
"We had understeer in the car but the rear wasn't there. As soon as I touched the throttle at any speed it was gone, the rear. At both high speed and low speed, it wasn't normal.
"I don't know why that is. We have to look at it more thoroughly."
McLaren had hoped that updates introduced for the Spanish GP would lift its performance, as Button warned that earlier optimism about scoring points in Monaco may have been premature.
"After today, I don't think I expect points at all this year," he said. "But hopefully today was an off day. It didn't feel right.
"The weekend has been reasonably good for us and I have been happy with how the car has been. But it was like flicking a switch in the race.
"It was not the easiest – it was from the word go. It is not a balance issue because I also had understeer in the car too."
Button's teammate Fernando Alonso was forced to retire from the race with brake problems.