Labour's Port Hills MP, Ruth Dyson, will retire at next year's election.
By then she will have been in Parliament for 27 years.
"This has not been an easy decision to make but it is one that has been made considerably easier by the fact that I am leaving at a time when Labour is strong and united, with a clear plan to deliver the policies that I have worked hard to develop," she said.
"I don't recall a time when Labour has been more united as a team or when we have had such an amazing recruitment of new activists."
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Advertise with NZME.Dyson is aged 61 and was first elected in 1993.
Currently chief Government whip, she has been a former Minister of ACC, Labour, Disability Issues, Social Development and Women's Affairs.
She was also Labour Party president during a tumultuous time, 1988 to 1993, when the party was bitterly divided over its economic reforms.
She resigned her ministerial portfolios in 2000, just a year after the Helen Clark Government had been in power, because of a drink-driving offence but she was reinstated the following year.
Prime Minister Jacinda Ardern paid tribute to Dyson saying she had been "a loyal member and outstanding contributor to the Labour family her entire adult life."
"As president, MP, minister and whip she has held just about ever major role in the party.
"She is smart, generous and kind. We are going to miss her.
"As with her entire political career she will go out in her own way, well signalled and planned and achieving that rare thing in politics where your leaving is of your own choosing."
