BMW chief Mario Theissen has admitted the manufacturer's Formula One ambitions were being restricted during their partnership with Williams.
The German company are pulling out of their engine-supplying arrangement with Williams to concentrate on running their own team from 2006.
BMW bought out Swiss team Sauber in June and will take over their operation and place on the grid next year.
Theissen said: "An optimal overall package can only be achieved with a fully integrated team and coherent processes throughout.
"For the past five years we pushed with Williams but never felt that we fully accomplished.
"That was when we started to evaluate the option to create our own team – to have full control of all success factors."
BMW's relationship with Williams had been deteriorating before the decision to buy Sauber was announced.
Team boss Sir Frank Williams had openly criticised the manufacturer but Theissen insists they will not throw the towel in on the current season.
He added: "There is still one third of the season to go and Frank and myself are pushing very hard to close the gap to the front-runners.
"Success is the best way when it comes to a goodbye. We want to finish the partnership on a high, not on a low. And we are all working very hard to achieve this."
