Mercedes didn't believe sensors over Bottas's debris strike

Bottas was leading at Imola on lap two when he collected a piece of Sebastian Vettel's endplate, which had broken off after the German collided with Kevin Magnussen at Tosa corner on lap one.

While Bottas had avoided the risk of a puncture by running over the debris straight on, the broken part struck the underside of his Mercedes before getting stuck.

Mercedes telemetry instantly signalled to the pit wall that Bottas was suffering a performance deficit, but the team at first could not believe how big the problem was because the Finn was still holding on to the lead.

Andrew Shovlin, Mercedes' track engineer director, said: "We were seeing a big drop in performance, but the issue is the scale of the drop in terms of laptime. If you translate it to lap time, it was seven or eight tenths of a second.

"We weren't really believing the sensors, because we were looking at how Valtteri was not pushing particularly hard, and able to do reasonable lap times when we needed to. We could build a bit of a gap.

"And it's very difficult to put an absolute lap time loss on these because they can often affect the car in a quite non-linear way around the circuit, or different behaviour in different directions and corners. But early on, we couldn't quite believe how big it was."

Mercedes knew that Bottas had hit the debris at Tosa corner, but where Vettel's endplate had got stuck – and the fact that only its black surface was visible – meant it could not fully understand what was causing the problem.

Shovlin added: "We didn't know what it was. When you look at the size of the bit, and the fact that it's red, you can't believe that we couldn't see it on the cameras.

"But it was lodged in, and it was the black area that was visible. And because the whole car's black there, we couldn't see that. If we'd been able to see where it was, we could have possibly done a more effective job of getting it out for the first stop.

"It's unlucky for him and he definitely made the right decision to go straight over it because taking that onto a tyre would very likely have caused an instant puncture. So that was a good call. But through no fault of his own, it basically held him back for the whole afternoon."