Valtteri Bottas says he wasn't aware he was driving on a mis-matched set of tyre compounds during his second stint of the Belgian Grand Prix until he was informed he had to serve a drive-through penalty for the blunder.

Having qualified in third position, Bottas was already enduring a disappointing race after slipping down to seventh in the early stages, only for it to get worse when a mix-up at the first pit-stop saw the team fit a medium right-rear tyre amongst a set of soft tyres.

Landing him a drive-through penalty as a result, Bottas admits he wasn't aware there was even a problem until he asked why he was being called in, saying the car was surprisingly 'driveable'.

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"They told me the lap after my penalty when I asked what the problem was," he said. "I thought initially there was just a bit of a mistake with the tyre pressures because the corners to the left felt a bit different. It was still driveable, I mean I was not losing massive time because of that. Obviously a very disappointing mistake from us."

Going on to recover some points in ninth place, Bottas says he was disappointed that Williams didn't prove as strong in the race as many had expected.

"We struggled today with the option tyre and the prime we thought was going to be the worst one but actually it was the better one for us. So we need to look in to that. I really hope we are stronger there and I do expect us to be also."

The result means Bottas has slipped from fourth to sixth in the overall standings, though just three points split himself, Felipe Massa and Kimi Raikkonen.