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Labour MP Jamie Reed is to resign as an MP at the end of January.

The MP for the marginal seat of Copeland in Cumbria will give up his seat to take a job in the nuclear industry - triggering a by-election.

He will become Head of Development and Community Relations for nuclear decommissioning firm Sellafield Ltd.

Announcing his resignation on Twitter, he said serving as an MP had been "a privilege beyond compare. Thank you all."

Mr Reed has been one of Labour leader Jeremy Corbyn's most persistent backbench critics.

He called for Mr Corbyn’s resignation in June, in a scathing letter of resignation from his Shadow Cabinet.

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“In the short time you have been Leader, along with the shadow Chancellor, you have sought to inject an unprecedented poison into our party,” Mr Reed wrote.

“You have actively worked to divide Labour MPs from the Labour Party membership…you have repeatedly incited you supporters on social media to confront Labour MPs; without doubt you are not fit to lead our great party.”

But today, in a letter to Mr Corbyn revealing his decision, he said: "I wish you every success in your endeavours to become our next Prime Minister."

Of his new job, he said: “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 said he was proud of his achievements over 12 years as an MP, and of the last Labour government.

He wrote: “Together we built schools, town centre developments, university facilities, dental surgeries, healthcare centres and a new West Cumberland Hospital of which I am most proud.

“This is the hospital where my four children were born and which saved my life in 2010 when admitted to intensive care as a result of undiagnosed Type 1 diabetes.”

He added: “The ambitions of Copeland and other communities like it are always best served by a Labour government that seeks to renew our nation and which redistributes power, wealth and opportunity.

(Image: Reuters)

“I have long argued that our national politics must meet this challenge so that communities like mine - far removed from our major urban centres - can shape our own futures. I hope that the next Labour government will help to deliver this.

“Now that our country has voted to leave the European Union, we need a Brexit that works for communities like Copeland, and one that works for the whole of our country.

“I love my community and I love the Labour Party. Leaving Parliament is the hardest decision of my life.”

Mr Reed held a 2,500 majority in the 2015 election, a 3.8% drop on the 2010 election.

Both the Labour and second-place Tory candidates in Copeland lost significant ground to Ukip - whose vote share increased by some 13.2% to come from nowhere in to third place.

Jamie Reed's resignation letter in full

Dear Jeremy,

It has been an honour to have been elected to represent the people of Copeland in Parliament at the last three general elections as a Labour MP. It has been a privilege without compare to serve the community that I was born and raised in.

From the end of January 2017, I will cease to serve as the Member of Parliament for Copeland in order to join Sellafield Ltd as its new Head of Development & 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.

I am proud of what I have achieved over the past 12 years with the help and support of the Copeland community and the last Labour government.

Together we built, schools, town centre developments, university facilities, dental surgeries, healthcare centres and a new West Cumberland Hospital of which I am most proud. This is the hospital where my four children were born and which saved my life in 2010 when admitted to intensive care as a result of undiagnosed Type 1 diabetes. Universally accessible, high quality local health services, free at the point of use must always remain central to our political mission

West Cumbria now has its best days before it and - thanks to all our hard work - Copeland will soon become one of the fastest growing areas of the UK economy. I look forward to being part of this development in my new role.

I am indebted to all those who have supported, advised, assisted and encouraged me over the years. I am particularly grateful to the Copeland Labour Party and the hardworking, dedicated, decent people who sustain it.

In our country and in my community, the Labour Party remains the greatest movement for social and economic progress we have ever seen or are ever likely to see.