Labour MP Jamie Reed is to quit Parliament, triggering a by-election in his Copeland constituency, he has announced.

Mr Reed, who has represented the Cumbria seat since 2005, announced in a letter to party leader Jeremy Corbyn that he will down at the end of January 2017.

He will take a new job at nuclear power station Sellafield as the company’s new Head of Community Relations.

“This is an important role that will build upon my work in Parliament and my previous time at Sellafield and allow me to continue to help the community and industry of West Cumbria to meet the challenges of the future,” he wrote.

At the last election Mr Reed won his seat with 16,750 votes to his Conservative rival’s 14,186, with Ukip in third place on around 6,000 votes.

Though the seat has elected a Labour MP since its creation in 1983, the ensuing by-election will likely be a test of Mr Corbyn’s leadership, with current national polls suggesting it may not be invulnerable to capture by another party.

Mr Reed has been a public critic of Mr Corbyn, describing himself light-heartedly as “Red Leader, Rebel Alliance” in his Twitter bio – a reference to the Labour rebels and to the Star Wars film franchise.

He refrained from direct criticism of Mr Corbyn in his resignation letter, saying only that Copeland “desperately need[s] a Labour government so that communities like mine can build the future they deserve”.

This summer he called for Mr Corbyn to quit, saying that the leader and those around him had “chosen to poison the well of our national political discourse”.

“You will never be the leader of our country and have no ambition to do so,” he said in a letter at the time.

Before his election Mr Reed also worked for Sellafield, which is based in his constituency.

In his letter today he added: "I love my community and I love the Labour Party. Leaving Parliament is the hardest decision of my life."

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In a statement, Mr Corbyn said: "I am sorry to hear that Jamie Reed is to stand down as MP for Copeland.

“He 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.