Valtteri Bottas said Williams is unsure what caused the damage to his engine cover which left a big hole in the bodywork during FP2 for the British Grand Prix.

Having sat out the first session, Bottas had completed the most laps of any driver in second practice when something appeared to explode under the engine cover as he accelerated down the Hangar Straight. Speaking immediately after the session, Bottas said there was no obvious cause of the damage.

"It will be investigated so that we are sure it won't happen in the future," Bottas said. "I don't know [what caused it] yet, we will have to see but it just came off really."

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And Bottas was encouraged with the progress he was able to make during FP2 after missing the morning session.

"Not bad for just one session. It was quite tricky conditions today for just one session and it's not an easy place to set up the car so there is still work to do for tomorrow. It looks like Ferrari and Red Bull are a bit closer than in Austria so I think it's going to be really tight. Anything that we can improve is going to help."

With neither Susie Wolff nor Felipe Massa completing many laps in FP1, Bottas said the team would look to recover in the post-race test.

"We missed some aero tests that we had planned for the morning so it's a pity. The main thing that we focused on in practice two then was just the set-up work and then the aero tests we can always do next week in the test here. So it's one session really lost for us but we can't change it."

Although Friday's running took place in dry conditions, Saturday is forecast to be wet but Bottas doesn't expect Williams to have any issues in the rain as it had earlier in the season.

"I think the improvements we made for the wet should be still there. The car has improved since we last drove in the wet so I am confident that we should still be quite good."