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A fierce campaign centred on the Tories’ planning reforms and the construction of the HS2 rail line saw voters swing to Sir Ed Davey’s party by 25 points on Thursday, giving the Liberal Democrats 56.7 per cent of the vote.
The party is claiming to have punched a hole in the Blue Wall - a band of southern Tory-voting constituencies with large majorities. Sir Ed said the win had "sent a shockwave through British politics".
The result is one of the biggest swings in recent memory - but the UK has a history of producing dramatic results at by-elections.
Martin Baxter, of the political analysis firm Electoral Calculus, said by-election swings in the middle of parliamentary terms can suggest a “decay” in support for incumbent parties but are often driven by local political grievances.
“I have never found by-elections to be a good guide to predict future national outcomes, and they are driven by the special factors of the by-elections at the time,” he said.
“But the Liberal Democrats are quite good at holding onto particular by-election wins once they have had them.
“It doesn’t mean that every seat other than Chesham and Amersham is now going to fall to the Liberal Democrats.
“When the Government starts to decay, as it were, they begin to lose everywhere, against every party for many reasons.
“The Conservatives had a very strong showing in the local elections. This is really the first electoral setback for the Conservatives in a while [...] so this is one swallow.
“I guess the opposition parties will be hoping that more swallows will arrive and herald the summer, but at the moment this is just one data point.”
2014: Clacton and Rochester and Strood
A double whammy of by-elections in Clacton and Rochester and Strood in 2014 scared the Conservative Party into getting serious about Brexit after it lost two seats to UKIP. Both votes were triggered by the defection of incumbent MPs - Douglas Carswell and Mark Reckless - to Nigel Farage’s party.
When the pair stood for re-election as UKIP candidates in their Leave-supporting seats, both returned to the Commons. Mr Carswell’s campaign produced a 44-point swing, while Mr Reckless’s swung by 28 points.
The results suggested a national swing towards UKIP and the cause of Brexit. The results came the year after David Cameron offered Britain an in/out referendum on Europe against a national backdrop of increasing Euroscepticism.
The following year, after winning the 2015 general election, Mr Cameron’s party fired the starting gun on the referendum campaign that ultimately ended his premiership and brought the UK out of the EU.
Clacton is now one of the safest Tory seats in the country.
2012: Bradford West
A by-election in a former Labour stronghold in Bradford in 2012 saw the shock victory of George Galloway for the Respect Party, with a 36-point swing.
Mr Galloway’s party had been a fringe socialist outfit with little expectation of him performing well. After the result he declared there had been a "Bradford Spring" - an analogy with the Arab Spring that was taking place around the same time.
The result reflected concern in the local Asian community with the local Labour Party. Mr Galloway ran a campaign directed at the large proportion of Muslim voters in the area, who formed the bulk of the swing in his favour.
Ed Miliband, the Labour leader at the time, said his party needed to "learn lessons" from the defeat, but that it had been decided by "local factors that were particular to the constituency".
The sixth-largest by-election swing in modern British history came in the southern constituency of Christchurch in 1993.
Diana Maddock, the Liberal Democrat candidate, presided over a 38-point swing away from the Conservative Party in an historic Tory stronghold, taking almost double the votes of John Major’s party.
The result was seen as an indication that the Tory Government had lost its appeal under the leadership of Mr Major, even in what looked like a safe seat. The by-election came just months after the Black Wednesday debacle of September 1992, which is generally viewed as the low point of his premiership.
But despite the crushing blow to the Tories in 1993, the seat swung back to the Conservatives at the next general election in 1997 - bucking a national trend of increased support for Tony Blair’s New Labour.
The new Christchurch MP, Christopher Chope, remains the MP for the constituency with a majority of almost 25,000.
A vicious campaign between Simon Hughes and Peter Tatchell in 1983 produced the biggest by-election swing in living memory, with voters shifting away from Labour by 44 points.
Much of the campaigning against Mr Tatchell, who stood for Labour, was homophobic and Mr Hughes later apologised for the abuse he received.
But the result also reflected broader anxieties in the electorate about the hard-Left sections of the Labour Party. Mr Tatchell was identified by many including his party leader, Michael Foot, as an advocate of violent action after writing in support of civil disobedience in a political magazine.
The swing in favour of the Liberal Democrats was maintained until 2015, when a version of the constituency with redrawn boundaries fell to Labour’s Neil Coyle. Mr Hughes ran twice for the leadership of his party and to be Mayor of London but was unsuccessful.
2003 - Brent East
A by-election in the Labour-held London constituency of Brent East in 2003 was another good night for the Liberal Democrats, after the party took almost 40 per cent of the vote share on a 29-point swing.
Unlike in Christchurch a decade previously, the Liberal Democrats under Charles Kennedy managed to hold onto the seat at the next general election in 2005.
The timing of the by-election in a Labour-helds seat suggested many voters were unhappy with the Blair government over the UK’s involvement in the war in Iraq.
Mr Kennedy hailed the beginning of a “very different Britain” in his conference speech that followed the by-election - but strong Lib Dem gains would not ultimately materialise until the 2010 election that brought Nick Clegg into power in coalition with David Cameron’s Conservatives.