Batley by-election to be on 20 October
By-elections after the death of Labour MP Jo Cox and the resignation of former Conservative Prime Minister David Cameron will be held on 20 October.
Mrs Cox was killed in her Batley and Spen constituency in June.
Voters in Witney will also elect a new MP to replace Mr Cameron, who stepped down from Parliament on Monday weeks afters resigning as PM in the wake of the UK's vote to leave the EU.
The writs for both by-elections were moved in the Commons on Thursday.
Mrs Cox, 41, who was the first sitting MP to be killed since 1990, won the seat with a comfortable majority of 6,057 at the 2015 general election ahead of the Conservatives in second, and UKIP in third.
The Conservatives, Liberal Democrats and UKIP have said they will not take part in the Batley and Spen contest out of respect for Ms Cox's memory.
The English Democrats and Liberty GB have said they will.
Witney has been one of the Conservatives' safest seats, with Mr Cameron securing a 25,155 majority at the last general election.