Diane Abbott has said she will quit the front bench of Labour when a new leader is elected.
Ms Abbott, the shadow home secretary, told Sky News that whoever becomes leader, "they have to be able to construct their own shadow cabinet".
Earlier Jeremy Corbyn had said he would consider a senior post under the new leader.
Ms Abbott said she would be backing Rebecca Long-Bailey as her choice for Labour's replacement leader.
Three candidates are in the running for leader: Ms Long-Bailey, Lisa Nandy and Sir Keir Starmer.
Voting begins on Monday, with the new leader announced on 4 April.
Ms Abbott, MP for Hackney North and Stoke Newington, said she would be returning to the backbenches "where there's an awful lot to do".
Ms Abbott was the first black female to be elected to Parliament in 1987.
She made history in October 2019 by becoming the first black MP at the despatch box at Prime Minister's Questions, in place of Mr Corbyn.