The teams had already signed a commercial agreement in March.
The future of Formula One was secured on Saturday after the governing body (FIA) and the 12 teams signed a new Concorde Agreement to safeguard the sport until 2012.
Swiss-based Sauber will take departed Toyota's place as Formula One's 13th team next season, the governing International Automobile Federation (FIA) said in a statement on Thursday.
Max Mosley backed former Ferrari boss Jean Todt as his ideal successor on Wednesday after ending speculation that he would seek a fifth term as head of motorsport's world governing body. The International Automobile Federation (FIA) president wrote to all member clubs saying he had received almost 100 messages of support from within the organisation but had decided to stand down anyway.
New Formula One teams US F1 and Campos will breach regulations if they are not ready in time for the season-opening race in Bahrain on March 14, the International Automobile Federation (FIA) said on Wednesday.
Williams join Ferrari, Midland/Jordan, Red Bull and Toro Rosso/Minardi in agreeing to an extension with the F1 chief.
Red Bull joined champions Ferrari in agreeing to extend Formula One's existing commercial agreement to 2012 and turning their backs on a rival series.
The FIA is concerned that cars are taking corners far faster than in the past, with clear safety implications.
Renault, Honda, Toyota, McLaren-Mercedes and BMW-Sauber will not be guaranteed one of the 12 team slots if they failed to sign up in time.
"Mexico's planned return to the Formula One calendar next year is looking doubtful," said FIA president Max Mosley.
The strangest of campaigns crammed 17 races, instead of the record 22 originally envisaged, into little more than five months from July with four triple-headers.
Struggling smaller F1 teams like Sauber and Force India have long argued that the distribution is skewed in favour of leading manufacturers, who receive special payments regardless of how they perform on the track.
Credit rating agency Moody's changed Formula One's outlook to negative from positive on Thursday to reflect the impact of the coronavirus crisis on a global sport that has yet to race in 2020.