McLaren made Williams an offer of "majestic proportions" for German driver Nico Rosberg last year but it was rejected immediately, team boss Frank Williams has revealed.

"It was never a thought we entertained," Williams told reporters at a pre-season lunch on Friday. "We had an offer...of majestic proportions but it was never ever discussed.

"Why would we give away the crown jewels? Why bother improving the car by one second if we then give away half a second with the driver?"

Co-owner Patrick Head confirmed the approach from McLaren came after the end of the season when Spain's double world champion Fernando Alonso had already left the Mercedes-powered team.

McLaren eventually signed Finland's Heikki Kovalainen from Renault in mid-December to partner championship runner-up Lewis Hamilton.

Rosberg, son of Finland's 1982 world champion Keke, had earlier signed an extension of his Williams contract to take in 2008 and 2009.

"I was a little surprised he was happy to sign to stay," said Williams. "I feared he would go to McLaren and I was a bit surprised that he didn't.

"He could not have gone there this year, he had a fixed contract, but 2009 was going to be the worry. We had an option for 2009 but it was very tough terms, based on the constructors' championship."

Toyota-powered Williams finished last season in fourth place overall but would have been fifth had McLaren not been stripped of all their constructors points for a spying controversy involving leaked Ferrari data.

Rosberg and his new Japanese team mate Kazuki Nakajima, also the son of a former grand prix racer, have been setting impressive times in pre-season testing and the German said the 2008 car was a big improvement.

"They've done a really good job and we've really progressed but the others have too, so we'll see," said the 22-year-old.

"I think we are going to progress this year and fourth in the constructors' has to be the goal.

"I don't think BMW can be beaten but to beat all the rest, Renault and Red Bull," Rosberg added, placing champions Ferrari and McLaren in a class of their own.

"I would be happy if I could be fighting for fifth place. If I don't have to depend on other teams dropping out to finish fifth, that would be pretty good...and then with some luck to get a podium."

The new Formula One season starts in Australia on March 16.