A two-minute silence will be held in memory of murdered Labour MP Jo Cox during Thursday's by-election count in Tooting.
Voters in south London are choosing a replacement for Labour's Sadiq Khan, who stood down when he was elected as the city's mayor.
The silence will be held during the count, when Labour councillor and junior doctor Rosena Allin-Khan will battle to hold the constituency for her party against Conservative Dan Watkins.
Paul Martin, acting returning officer, said: "The murder of Jo Cox has shocked the entire country, and it has hit home particularly hard in Tooting where right now the community is in the process of selecting a new member of parliament to represent their interests.
"It is our duty and responsibility to continue with this by-election, but clearly a very dark shadow has been cast over these proceedings.
"The flag over Wandsworth Town Hall is now flying at half mast and we will hold a two-minute silence during this evening's count to reflect upon this awful loss.
"Our thoughts, prayers and deep condolences are with Jo's family and loved ones at this incredibly difficult time."
Khan, who had been the area's MP since 2005, held Tooting with a majority of 2,842 over Watkins - a local businessman - at the 2015 general election.
The by-election result will be closely watched by Jeremy Corbyn's critics in the Labour Party, but coming a week before the EU referendum, the campaign has been overshadowed by the closing stages of the Brexit battle.
Born and raised in the area, Dr Allin-Khan works as an emergency doctor at St George's Hospital and is also deputy leader of Wandsworth Labour group.