Bottas, who had started in third place, was handed a drive-through penalty after the team gave him three soft tyres and a medium at his first stop — an error evident to television viewers given that the tyre walls are colour-coded.
“As a team we have made a mistake, and on behalf of the team I am very sorry that we cost him what could have been a podium position,” said performance engineering head Rob Smedley.
“There will be an investigation into the processes in place to understand what happened fully, and to put in place a procedure to stop this happening again.”
Bottas ended up ninth, with Brazilian Felipe Massa finishing sixth.
“The pitstop and the race pace are two areas that we need to investigate,” said Bottas.
“We thought we would be stronger during the race but the soft tyre in the first stint didn’t work as well and put us in a difficult position. We have to make sure we bounce back well in Monza.”
Williams had hoped for far more at a circuit that favoured their Mercedes engine and in the sort of ideal dry conditions that rarely occur at Spa with its notoriously capricious climate.
Smedley said the first stint dictated the result, with both Williams drivers struggling for pace.
The only consolation for Formula One’s third-placed team was that rivals Ferrari and Red Bull only had one car each in the points.
“We have gained on Ferrari and lost very few points to Red Bull which is positive considering the day we have had,” said Smedley.
Williams have 161 points to Ferrari’s 242 and Red Bull’s 108. Championship leaders Mercedes are way ahead on 426.
(Reporting by Alan Baldwin, editing by Ken Ferris)