UK 21 December 2016 Labour faces biggest by-election test after Jamie Reed's resignation The party held Copeland by just 2,564 votes in 2015. Getty Images. Sign UpGet the New Statesman's Morning Call email. Sign-up One minute into Jeremy Corbyn's 2015 Labour leadership victory speech, Jamie Reed MP resigned as shadow health minister. Among the reasons he cited was Corbyn's opposition to nuclear power, a major industry in his Copeland constituency. Today, Reed announced he is standing down from parliament after 12 years to take up a job at Sellafield. Though he has been one of Corbyn's most vociferous critics, he said the Labour leader was not the cause for his resignation. “This decision has got absolutely nothing to do with Jeremy Corbyn. On a personal level I’ve got a very cordial relationship." Reed, who will become Sellafield's head of development and community relations on 1 February, added: "One of the reasons that I am moving on is because I think there’s a better way of doing that [serving his community] right now than to remain as a member of parliament." The main political consequence of Reed's departure will by a by-election in Copeland - the most marginal constituency to have been contested since May 2015. Labour has held every existing seat it has fought since Corbyn's election but this will be by far the greatest test. Reed was re-elected with a majority of just 2,564 and the Conservatives are in second place. The constituency was 62 per cent Leave in the EU referendum and Ukip finished third in 2015 with 6,148 votes. If, as in other recent by-elections, Labour's vote is squeezed from all sides, it could lose. Indeed, for all of the main parties, Copeland is a defining test. Corbyn must prove that Labour can defend marginal seats from the Conservatives. Theresa May must prove that the Tories are widening their appeal. Ukip must live up to its vow to "replace" Labour. And the Liberal Democrats must show they can thrive as the party of Remain. The people of Copeland should brace themselves for an onslaught of campaigning. › Is this the end of democracy? George Eaton is senior online editor of the New Statesman. Subscribe For daily analysis & more political coverage from Westminster and beyond subscribe for just £1 per month!