Cat Smith said she was stepping down as shadow secretary of state for young people and democracy.
Writing to her party leader, Ms Smith - the MP for Lancaster and Fleetwood - said she was "grateful" for Sir Keir's offer to keep her in her shadow cabinet position.
However, she said she would leave her role, adding: "I wish to focus more of my time in my Lancashire constituency."
Corbyn row
She said the pair had a meeting scheduled during which she wanted to raise concerns about former leader Jeremy Corbyn "not being readmitted to the Parliamentary Labour Party".
She wrote that the current situation was "utterly unsustainable" adding "it is important you truly understand how much damage this is causing".
In her letter, Ms Smith also raised the issue of proportional representation, and concluded that she wished the shadow cabinet "well" but added: "I do hope you will reflect on some of these concerns I have aimed to outline constructively and from the point of view of one of our few remaining northern 'red wall' Labour MPs."
Last week, the Labour leader told the BBC he had not spoken to his predecessor Mr Corbyn in over a year, since a report was published into anti-Semitism in the party in October 2020.
Mr Corbyn's reaction to the report saw him suspended from the party - and while he was allowed back as a party member, he still sits as an independent MP
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