She confirmed the news on Tuesday morning by publishing her email to Camberwell and Peckham Labour Party members on social media.
Ms Harman wrote that it had been an “overwhelming honour” to be an MP “representing and working for the people of Camberwell and Peckham for nearly 40 years”.
As the longest-ever continuously serving female MP in the House of Commons, Ms Harman was given the nickname “Mother of the House” by prime minister Theresa May in 2017.
She has worked with eight different Labour leaders and under seven different prime ministers. She has also won her London seat eleven times since first taking it in a 1982 by-election.
Under Tony Blair, Ms Harman was secretary of state for social security and the first minister for women.
In her statement she wrote: “I entered the Commons as one of only 11 Labour women MPs in a parliament that was 97 per cent men. Now there are 104 Labour women and across all parties women MPs are a ‘critical mass’.
“But there remains much more to de done till women genuinely share political power with men on equal terms and until women in this country are equal.”
She continued: “I will leave the House of Commons with my feminism, my belief in Labour and my enthusiasm for politics undimmed.”
Ms Harman has held a raft of posts within the Labour party, including two brief stints as leader after Gordon Brown and Ed Miliband stepped down.
In her email she said she remained proud of the achievements of the Labour government: referencing the introduction of the national minimum wage, the right to civil partnership, Sure Start children’s centres and the Equality Act.
Two other long-serving Labour MPs, Barry Sheerman and Dame Margaret Hodge, have also announced their plans to step down at the next general election.
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