Mario Theissen accepts the FIA International Court of Appeal's decision to rule a disputed diffuser design legal.

But the BMW Sauber boss fears implementation of the component will cost teams "a lot of money."

The ICA on Wednesday sided with Brawn GP, Toyota and Williams that the double diffuser design those teams have used since the start of the season was legal.

Consequently, it means the remaining seven teams have to act quickly to make the necessary changes to their cars and be able to compete with the trio over the course of the season.

The decision brings to an end any doubts over the validity of Jenson Button's victories in the Australian and Malaysian Grand Prix in his Brawn GP car, but Theissen believes that the implementation of the diffuser will require a lot of investment in a time when Formula One is on a cost-cutting drive due to the current financial climate.

"We are not happy," said Theissen. "We cannot be happy with the decision but we will definitely accept it.

"It is very difficult to switch to a diffuser concept like this. You cannot simply put it on the car, we have to redevelop the aerodynamic concept of the car

"We have to work and find a solution ourselves. It will take time and a lot of money. We have started development but so far we don't have a result."

BMW Sauber's cause will not be helped by the new rule brought in for this season that forbids any testing between races.

"If you have to change the concept of the car during the course of the season you need to test it," added Theissen. "You can hardly simulate it.

"You have to go to the track and this opportunity will be more difficult."