Elsewhere, in Stoke, Labour's Gareth Snell won with a reduced majority of 2,620 over nearest rivals UKIP - but it's the Copeland result which has really turned heads. There's no understating the scale of the defeat - the worst by-election performance by an opposition since 1878 by some measures. The Tories argued that Corbyn was hostile to nuclear power, a large source of local jobs.
There was relief for Labour as the party saw off a challenge from Ukip leader Paul Nuttall in Stoke-on-Trent Central, although its vote share slipped in the previously safe seat.
A senior member of Jeremy Corbyn's top team has urged him to listen to the public and not shy away from "painful conclusions" of the "devastating" Copeland by-election result.
But arguably, it was only because Nuttall self-sabotaged himself - without even knowing about it, through a campaign that showcased a litany of scandals - that allowed Labour to recoup some its votes.
"I was elected to lead this party, I was elected to oppose austerity and to oppose the redistribution of wealth in the wrong direction, which is what this Government is doing". The Conservatives are treading a new path between nationalism and globalization, with a healthy strain of populism.
John Woodcock, the Labour MP for neighbouring Barrow and Furness, said the result was "a disaster". That it needed more of what Maurice Glassman used to call "flag and family".
The Stoke seat, which Labour have held since it was established in 1950, became vacant after Tristram Hunt resigned from parliament in January. And this should be all the more worrying for Labour.
Copeland is home to the Sellafield nuclear power plant, and much was made of Jeremy Corbyn's anti-nuclear stance throughout the campaign.
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Downing Street said the Prime Minister, who campaigned prominently in both constituencies, was told of the victory in Copeland by text in the middle of the night. This is an odd depiction of the "Northern Powerhouse': indeed, the most recent Treasury document on that project forgot to mention Cumbria at all".
This defeat is the latest loss taken by Labour Party in what has been a stunning collapse of the party over the past decade.
Insiders at Labour HQ said the party had run robust and competitive campaigns in both seats, with adequate funding, strong candidates and a focus on Labour's key issues, highlighting that causes for the loss lay elsewhere. They weren't convinced that the party supported the nuclear industry...
Another statistic that makes uncomfortable reading for Jeremy Corbyn is that Labour's share of the vote has gone down in every by-election since the European Union referendum, with the exception of the Batley and Spen by-election, which was not contested by the Conservatives or LibDems following the murder of the Labour MP Jo Cox.
'From the point of view of someone who lives on an estate in Hull, his utopian, cosmopolitan, internationalist, anti-military non-patriotic beliefs are all what they already think of the Labour party.
But he was instead facing questions about his future last night after falling more than 2,600 votes short of winning a seat that, weeks earlier, had seemed within UKIP's grasp.
He said: "The Copeland meltdown is at Len McCluskey's door". Another source working with MPs frets that he can not see the floor to the Labour vote any more.
Triggered by the resignation of Jamie Reed, the Conservatives won a seat that Labour had represented for the best part of a century. The problem for the left is that Corbyn's hold on Labour is vice-like: for the party, it is proving to be a deadly embrace.
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