The 52-year-old, who has served as the MP for West Bromwich East since 2001, made the surprise announcement on the first day of campaigning. Mr Watson has served as the party’s deputy leader since 2015. His tenure has been marked by frequent clashes with Jeremy Corbyn, Labour leader, over the direction of the party.
Mr Watson has quarrelled with the leadership over Brexit: he is an advocate of remaining in the EU and has spoken at several events arguing for a second referendum. Friends of Mr Watson said he had “had enough” of factional Labour politics, particularly as Mr Corbyn’s control over the party has tightened.
In September, Mr Watson survived an attempt to oust him as deputy leader by allies of Mr Corbyn after the Labour leader intervened at the start of the party’s annual conference in Brighton.
Jon Lansman, founder of the pro-Corbyn campaign group Momentum, had proposed a motion at a meeting of the party’s ruling national executive committee seeking the abolition of the post of deputy leader. Mr Corbyn quashed the move, fearing it would overshadow the annual event.
In 2016, Mr Watson denied having any role in a failed attempt by scores of MPs to remove Mr Corbyn just hours after the EU referendum result, insisting that he was focused purely on his weekend at the Glastonbury festival. Allies of the leader were sceptical.
A one-time political officer for the AEEU, the engineering workers’ union, Mr Watson used to share a flat with Len McCluskey, general secretary of Unite the Union. But they became bitter enemies after Mr McCluskey used his political muscle to support Mr Corbyn from 2015.
One Labour insider said: “If Tom thought Jeremy Corbyn was fit to lead the Labour party and was going to win an election, he wouldn’t be leaving now. He’s done his best to rescue things but this just shows the position the party has got themselves into.
“I’m sure there will be others: with five weeks to go, people are going to be thinking about what they do.”
In a letter to Mr Corbyn on Wednesday evening, Mr Watson said “our many shared interests are less well known than our political differences” and stated he was leaving politics for personal reasons.
“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,” he wrote.
“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.”
In response, Mr Corbyn said he was “proud and glad to have worked with you over these four years”, adding that he hoped to continue working together in the future.
“Few people have given as much to the Labour movement as you have and I know that many thousands of members and trade unionists that you have inspired and worked with over the years will be very sorry to see you go.”
Labour party insiders suggested that Mr Watson could be followed by other centrist figures seeking to make a point on the direction in which Mr Corbyn is taking the party. One official close to the shadow cabinet said more “moderates” might quit before the election. “So far we’ve seen the Tories haemorrhaging some great MPs. Totally possible more of us could go to,” said one MP.
The departure of Mr Watson will be seen as the final capitulation of anti-Corbyn forces in the Parliamentary Labour Party in Westminster. Many Labour MPs are still resentful of the party’s sharp turn to the left since Mr Corbyn became leader in 2015, although most have abandoned any hope of unseating him.
Mr Watson was elected as deputy leader at the same time as Mr Corbyn seized the leadership four years ago. He comes from the relatively rightwing trade union side of the party and was previously an ally of Gordon Brown, the former prime minister. He was heavily implicated in the “curry plot” to bring down Tony Blair 12 years ago that made way for Mr Brown.
During his chequered career in parliament he made waves as a critic of phone-hacking journalists but was also condemned for pursuing false allegations of a paedophile ring among establishment figures.