The Liberal Democrat candidate in Canterbury has stood down because he feared dividing the Remain vote.
Tim Walker said he was concerned standing would allow the Conservative candidate to take the seat from Labour.
Mr Walker said the Lib Dems had tried to do a deal with Labour over the seat, and when that failed he made the decision himself to stand down.
The Lib Dems told the BBC they will be selecting a new candidate to contest the Kent constituency.
Labour’s Rosie Duffield took the seat from the Tories at the 2017 general election with a majority of only 187 votes.
The constituency voted 51% to leave the EU in the Brexit referendum in 2016.
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Writing in the Guardian, Mr Walker said: “Politics does not always have to be grubby and small-minded; sometimes it’s possible to acknowledge that what’s at stake is more important than party politics – and personal ambition – and we can do what’s right.
“In this invidious situation, both standing and not standing could be interpreted as weakness.
“But the nightmare that kept me awake was posing awkwardly at the count beside a vanquished Duffield as the Tory Brexiter raised her hands in triumph. I wanted no part in that.”
Speaking earlier on Tuesday evening, before the news emerged, Lib Dem leader Jo Swinson was asked about whether her party could still do an electoral deal with Labour,
She told the BBC Labour had failed to engage in previous talks with other Remain-supporting parties and said Labour “aren’t really qualified to be a part of a Remain alliance”.
Last week, the Liberal Democrats, Plaid Cymru and the Green Party announced an electoral pact, agreeing not to stand against each other in 60 seats in England and Wales.
On Monday, Brexit Party leader Nigel Farage announced his candidates would not be standing in the 317 seats won by the Conservatives in the 2017 election.