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The race to succeed Prime Minister Theresa May hasn't officially begun — but the field of contenders is quickly growing.

Health Secretary Matt Hancock said Saturday he is seeking to replace May, joining several others who have announced.

May said she will step down as Conservative Party leader June 7 and remain as a caretaker prime minister while the Conservative Party chooses a new leader in a contest set to officially kick off the following week.

Another contender, International Development Secretary Rory Stewart, said he could not serve in a Cabinet under Boris Johnson if the flamboyant former foreign secretary wins the top spot.

Stewart says he could not work for a leader who is comfortable with the idea of a "no deal" Brexit when Britain leaves the European Union.