Diane Abbott is standing in for Jeremy Corbyn at Prime Minister’s Questions at mid-day.
The shadow home secretary will be the first black MP to represent her party at the weekly Commons clash.
She will face Foreign Secretary Dominic Raab, who is standing in for Boris Johnson for the first time.
The PM is missing PMQs to deliver his keynote address to the Conservative conference in Manchester.
Ms Abbott said she was “looking forward to leading Prime Minister’s Questions” for Labour.
The Commons is normally in recess during the party conferences but MPs voted to keep sitting amid controversy over Mr Johnson’s unlawful prorogation of Parliament.
Traditionally, the leader of the opposition can ask six questions, however if the prime minister is unable to attend, both they and their counterpart are replaced by a senior frontbencher.
Among the backbench MPs also listed to put a question to Mr Raab on Wednesday are Labour MPs Teresa Pearce and Lucy Powell, as well as veteran Tory Eurosceptic Peter Bone.
Ex-minister Margot James, one of 21 rebels expelled from the Tory parliamentary party after rebelling against Boris Johnson in a bid to prevent a no-deal Brexit, is also down to ask a question.
Shadow foreign secretary Emily Thornberry and shadow business secretary Rebecca Long Bailey have previously stood in for Mr Corbyn.
In 2015 then-shadow business secretary Angela Eagle also replaced Mr Corbyn at PMQs.
Ms Abbott is a key ally of the Labour leader and has served in his shadow cabinet since 2016.
Making the announcement on Tuesday, Mr Corbyn tweeted: “This #BlackHistoryMonth we’re inspired by the struggles of black campaigners, including the first black MPs elected in 1987.
“Tomorrow one of those pioneering MPs, a child of the Windrush Generation, Diane Abbott, will be the first black person to represent their party at #PMQs.”