One of Jeremy Corbyn's most persistent backbench critics has announced he is to stand down from Parliament to take up a job in the nuclear industry.
Jamie Reed said he will quit as Labour MP for Copeland in west Cumbria at the end of January, when he becomes head of development and community relations at the Sellafield plant in his constituency.
His departure sets up a potentially awkward by-election for Mr Corbyn in a constituency which Mr Reed had held since 2005, but saw his majority over the Conservatives shrink to 2,564 in last year's general election.
In a letter to the Labour leader announcing his decision, Mr Reed said leaving Parliament was "the hardest decision of my life".
And he told Mr Corbyn: "We desperately need a Labour government so that communities like mine can build the future they deserve.
"I wish you every success in your endeavours to become our next Labour prime minister."
Mr Reed denied that his departure was sparked by his very open differences with the party leadership.
"This decision has got absolutely nothing to do with Jeremy Corbyn," he told The Guardian. "On a personal level I've got a very cordial relationship.
"One of the reasons that I am moving on is because I think there's a better way of (serving the community) right now than to remain as a Member of Parliament."
Mr Reed was on a list of MPs hostile to the leader drawn up by Mr Corbyn's campaign team during this year's leadership election. He voted for Trident renewal earlier this year, branding the leader's opposition to nuclear weapons "juvenile" and "narcissistic".
Nuclear issues may play a part in the Copeland by-election, as the constituency includes the fuel recycling and decommissioning site at Sellafield and neighbouring Barrow is the location for construction work on the Trident replacement submarines.
Mr Reed, who worked as a press officer at Sellafield before entering Parliament, said his decision to stand down was motivated in part by a desire to spend more time with his family, saying it was "very difficult being a long-distance dad".
His announcement prompted immediate speculation on social media that the Copeland by-election could provide a route back to Westminster for former ministers David Miliband or Ed Balls, fresh from his profile-boosting run on TV's Strictly Come Dancing.
Just minutes before Mr Reed's decision was made public, Mr Balls was asked on BBC Radio 2's Jeremy Vine Show whether he ever wanted to be back in the House of Commons.
He replied: "I never say never, because who knows? It's such a wild time in politics at the moment and I miss the purpose of being in politics."
But he added: "It's not my plan and it's not what I am expecting to do."
Mr Reed held Copeland with 16,750 votes (42%) in 2015, ahead of the Conservatives on 14,186 (36%), Ukip on 6,148 (15%) and the Liberal Democrats on 1,368 (3%).
Conservative chairman Sir Patrick McLoughlin said: "We are sorry to see Jamie Reed leaving Parliament, but congratulate him on his new job, and what must have been a very difficult decision for him and his family.
"It is a sorry state of affairs when talented Members of Parliament like him no longer feel comfortable representing Jeremy Corbyn's Labour Party - a divided party run by a leader who opposes Trident and wants to dismantle our armed forces.
"We look forward to the by-election, and the opportunity to set out how the Conservatives are building a country that works for everyone, not just the privileged few."
Mr Corbyn said local party members would choose a candidate to replace Mr Reed shortly.
"I am sorry to hear that Jamie Reed is to stand down as MP for Copeland," he said.
"Jamie has given great service to his party, his country and his constituency.
"He has been a powerful voice for rural and remote communities and served his community well, campaigning for additional flood defences and infrastructure for Cumbria. I wish him all the best in his future role.
"We now face a by-election and local party members will soon choose a candidate equally committed to Copeland and its people."
Ukip leader Paul Nuttall will not be standing in the by-election.
A spokesman for the MEP said: "Paul is focused on sorting out the party internally at the moment, so won't be standing.
"However, the party will be committed to fighting a strong campaign."