The 1991 Formula One season was the 42nd season of FIA Formula One motor racing. It featured the 1991 FIA Formula One World Championship, which commenced on 10 March 1991 and ended on 3 November after sixteen races. Ayrton Senna won his third and last Drivers' World Championship and McLaren-Honda won the Constructors' Championship. Senna's main title challenger was Nigel Mansell at Williams, who won five races on his return to the team compared to Senna's seven. Senna's fierce rival Alain Prost failed to win a race with Ferrari in stark contrast to his title challenge the year before and left the team even before the end of the season. 1991 also saw the debuts of future world champions Michael Schumacher and Mika Häkkinen.