The by-elections for the constituencies of the former Labour MP Jo Cox and former Conservative Prime Minister David Cameron will be held on 20 October.
Cox was the MP for Batley and Spen in West Yorkshire, before she was shot dead on 16 June 2016 in her constituency.
The constituency was won by Labour in 1997, and Cox became the MP in 2015. In respect for her memory, the Conservatives, Liberal Democrats and Ukip will not field a candidate in the by-election.
Labour has begun the process of selecting a candidate to stand.
Cameron, who resigned on 24 June, after Britain voted to leave the EU referendum, announced he would resign as an MP earlier this week. His constituency, Witney, is a safe Conservative seat in Oxfordshire.