The Liberal Democrats have won the Tiverton and Honiton by-election, overturning a Conservative majority of more than 24,000.
Richard Foord won the Devon seat with more than 22,000 votes after the constituency saw a swing of almost 30%.
The former Army major said it sent a “loud and clear message”.
The by-election was triggered when former Conservative MP Neil Parish resigned after admitting watching pornography in Parliament.
Mr Foord said the result was an “extraordinary and historic result which has sent a shockwave through British politics”.
“Tonight the people of Britain have spoken,” he said.
“They’ve sent a loud and clear message: it’s time for Boris Johnson to go, and go now.”
He told BBC Radio Devon he was “very humbled” by the result.
Analysis: Martyn Oates, political editor, BBC South West
The overnight annihilation of all the Liberal Democrat MPs in the South West in the 2015 general election was widely credited with winning David Cameron his slim majority and overturned a decades-long status quo in which the Lib Dems were the Conservatives’ equals and feared opponents.
The 2017 and 2019 elections saw a further downward spiral for the party which had held every seat in Cornwall just 10 years previously.
The once victorious Lib Dems were reduced to miserable third places and Conservative majorities grew and grew.
Then, in the 2019 local elections, the party enjoyed notable successes in Devon and Somerset.
This year they wrested Somerset County Council – soon to become a unitary – from the Conservatives.
Now comes this remarkable victory in Tiverton and Honiton.
If they can do that here – in a seat they’d never previously held – the Conservatives are bound to be wondering what they might capable of in seats like North Devon, St Ives and Yeovil which they’d held for decades until just seven years ago.
Liberal Democrat leader Sir Ed Davey said the “stunning win” should be “a wake-up call for all those Conservative MPs propping up Boris Johnson” and they “cannot afford to ignore this result”.
Mr Foord took the seat with 22,537 votes, beating his nearest rival, Conservative Helen Hurford, who polled 16,393.
The swing from the Tories to the Liberal Democrats saw Mr Foord secure a majority of 6,144.
The Local Democracy Reporting Service said Conservative candidate Helen Hurford was ushered out of the counting hall once the result was declared just after 04:00 BST, failing to give a concession speech or speak to the press.
Labour candidate Liz Pole, who came third, said people had shown “that they have had enough of the Conservatives”.
Mid Devon District Council said a total of 42,707 votes were cast and the turnout was 52.3% out of an electorate 81,661.
Full results of Tiverton and Honiton by-election
- Richard Foord (Lib Dems) – 22,537 (52.91%, +38.14%)
- Helen Hurford (Con) – 16,393 (38.49%, -21.72%)
- Liz Pole (Lab) – 1,562 (3.67%, -15.88%)
- Gill Westcott (Green) – 1,064 (2.50%, -1.34%)
- Andy Foan (Reform) – 481 (1.13%)
- Ben Walker (UKIP) – 241 (0.57%, -1.06%)
- Jordan Donoghue-Morgan (Heritage) – 167 (0.39%)
- Frankie Rufolo (FB) – 146 (0.34%)