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Labour has captured the constituency of Plymouth Sutton and Devonport, one of the most marginal in the country, from the Tories.
The seat, which was high on Jeremy Corbyn's target list, was held by the Conservatives with a majority of just 523.
However, Labour overturned that and won the seat by a comfortable margin of 6,002, winning 23,808 votes to the Tories' 17,806.
Conservative incumbent Oliver Colvile had been the area's MP since 2010 but will now be replaced by Luke Pollard, who was born in, and lives in, the city.
Voters in Plymouth Sutton and Devonport had earlier complained that they were being turned away from polling stations despite having their polling cards. Some were told they were not on the electoral register despite having registered.
One man, who has lived in the constituency for more than three decades and said he has voted at every general election, told the Plymouth Herald he had been told his name was not on the electoral register.
Anthony Prynn said the incident had left him “flaming” and said he had notified the council of the situation and was told to wait for a call back. “It is a joke,” he said.
“I waited for two hours for a call back, and have phoned five times altogether, but I haven't heard anything,” said Mr Prynn. ”They will have to have a re-election.“
UK Election Day 2017Show all 38 Door of Number 10 Downing Street is seen on the morning after Britain's election in London
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Caroline Lucas, co-leader of Britain's Green Party, speaks after retaining her seat in the general election, in Brighton
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Patrons watch the results for Britain's election in London
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Leader of the Liberal democrats Tim Farron celebrates beating Conservative party candidate James Airey (R) following the announcement of the results at the Westmoorland and Lonsdale constituency count at Kendal Leisure Centre on June 9, 2017 in Kendal, United Kingdom. After a snap election was called, the United Kingdom went to the polls yesterday following a closely fought election. The results from across the country are being counted and an overall result is expected in the early hours
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First Minister Nicola Sturgeon arrives at the Emirates Arena in Glasgow, as counting is under way for the General Election.
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Anna McMorrin hugs Jo Stevens MP after winning Cardiff North for Labour at the Sport Wales National Centre on June 9, 2017 in Cardiff, United Kingdom. After a snap election was called, the United Kingdom went to the polls yesterday following a closely fought election. The results from across the country are being counted and an overall result is expected in the early hours
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Vince Cable wins the Twickenham election for the Liberal Democrats. Richmond and Twickenham election
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Britain's Labour party leader Jeremy Corbyn rubs his face after arriving for the declaration at his constituency in London
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Emily Thornberry speaks after holding her Islington South and Finsbury seat at the Sobell Leisure Centre in Islington, north London
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Britain's Prime Minister Theresa May listens as the declaration at her constituency is made for in the general election in Maidenhead, England, Friday, June 9, 2017. British Prime Minister Theresa May's gamble in calling an early election appeared Friday to have backfired spectacularly, after an exit poll suggested her Conservative Party could lose its majority in Parliament
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Labour Party candidate Emily Thornberry smiles at a counting centre for BritainâÃôs general election in London
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Britain's Home Secretary Amber Rudd attends the count for her seat at a counting centre for Britain's general election in Hastings
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Ballots are counted at a counting centre for BritainâÃôs general election in London
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The London Eye is illuminated in blue, representing the Conservative Party, after the first exit polls announced in Britain's general election
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Election officials count votes in the Emirates Arena in Glasgow, Scotland
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The London Eye is illuminated in red, the color of the Labour Party, after first exit polls of Britain's national election announced in London
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BBC Television centre is illuminated with the exit poll results for Britain’s general election
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Labour’s Bridget Phillipson (centre) smiles after being announced as the winner of the election for the constituency of Houghton and Sunderland South at a counting centre
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Members of the election staff prepare to count ballots, in Theresa May’s constituency in Maidenhead
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Ballot papers are checked ahead of the count at the Westmoorland and Lonsdale constituency count at Kendal Leisure Centre
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Ballots are counted at a counting centre for Britain’s general election in Kendal
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Theresa May arrives to vote at a polling station in Maidenhead
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Jeremy Corbyn arrives to cast his ballot at a polling station in north London
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Lib Dems leader Tim Farron outside a polling station at Stonecross Manor Hotel in Kendal, Cumbria
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The leader of the United Kingdom Independence Party (Ukip) Paul Nuttall arrives to vote at a polling station in Congleton
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The error followed a postal vote mix-up that left more than 1,500 people unable to vote when their postal voting packs went missing.
The Plymouth result was the sign of a disastrous showing for the Tories, who have lost seats to Labour across the UK. With over two-thirds of seats having declared their results, the Conservatives are forecast to be on course to win 316 seats to Labour's 265.
That would leave both parties short of a House of Commons majority. In such a scenario, Theresa May would be given the first opportunity to try to form a government because she is leader of the party with the most seats.
If she is unable to form a government, however, Jeremy Corbyn could find himself on the path to Number Ten - possibly by forming a coalition with the SNP, Liberal Democrats and Green Party.
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