Tom Watson has quit as deputy leader of Labour dealing Jeremy Corbyn a blow as the party's General Election campaign kicks into gear.
Mr Watson has said he made the decision to spend more time with family and the move is "personal, not political".
He said he will still campaign over the coming weeks and only formally leave the role of deputy leader on December 12.
Labour will be forced to field a new candidate in Mr Watson's vacated seat of West Bromwich East, which he has held since 2001.
His letter to Mr Corbyn tonight said: "Serving the Labour Party has been the privilege of a lifetime. I joined in 1982 and never imagined that one day a kid from Kidderminster would be the party’s Deputy Leader.
"Most importantly for me, for the last 18 years it’s been my immense honour to represent people from Sandwell. It’s hard to find words to express my deep gratitude to the people who trusted me to fight their corner."
He addd: "But now is the right time for me to stand down from the House of Commons and start a different kind of life. The decision is personal, not political.
"The last few years have been among the most transformational of my personal life, second only to becoming a proud father of two beautiful children. I’ve become healthy for the first time, and I intend to continue with this work in the years to come.
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"I want to thank you for the decency and courtesy you have shown me over the last four years, even in difficult times."
In response, Mr Corbyn thanked him for his contribution to the party and said he had respected his decision.
He also said he hoped the "horseradish plant" he had given him as a gift thrived. Both men commented on their shared love of horticulture.
Mr Watson was elected deputy at the same time Mr Corbyn scooped the top job in 2015.
But his tenure has not been without controversy – he faced calls to resign after giving vocal support to the false VIP paedo ring claims made by the fantasist Carl Beech, who was originally known as "Nick".
He even said there was "clear intelligence" of a powerful paedo network linked to parliament and Number 10.
He also repeatedly clashed with Mr Corbyn's hard-left supporters who tried to remove him, and disagreed with the leader on some policy issues.
Tonight, opening the Tory campaign in the West Midlands he said it would be the "most important election in a generation".
The Prime Minister said he had been forced to go to the country because Parliament was "paralysed" over Brexit.
He said MPs had become incapable of functioning "like an anaconda which had swallowed a tapir".
"For three-and-a-half years we have had non-stop political manoeuvring to stop Brexit and thwart the will of the people," he said.
Tonight Daniel Janner, son of the late Labour MP Lord Janner, who was one of the victims of Beech claims, said Mr Watson's position had become "untenable".