Nigel Farage’s Reform UK has dramatically won the Runcorn and Helsby byelection by just six votes in a blow to Keir Starmer’s premiership.

The hard-right party narrowly overturned Labour’s 14,700-vote majority in the first full-scale electoral test of Starmer’s government and set a new record for the smallest majority at a parliamentary by-election since the end of the second world war.

The result, which came on a night when Reform UK was expected to gain hundreds of council seats across England, followed a 17% swing from Labour to Farage’s party.

After a recount that delayed the declaration by three hours, Reform’s Sarah Pochin won 38.6% of the vote – amounting to 12,645 votes, six more than Labour, making it one of the smallest margins of victory in recent UK political history.

The Conservatives slumped from 16% of the vote at last year’s general election to 7% in this contest, narrowly finishing ahead of the Green party in third place.

The closely watched contest had been billed as the first real test of Farage’s ability to turn his party’s rising popularity into seats in parliament.

Pochin, a former Tory councillor and local magistrate, becomes the first non-Labour MP to represent the Cheshire town of Runcorn in 52 years.

Arriving at the count centre to declare victory at 6am on Friday, Farage said: “Here and across the country you’re seeing big swings to us, from Labour in the north and Conservatives in the Midlands and the south. It’s fascinating”.

He added: “For the movement, for the party, it’s a very, very big moment, absolutely no question and it’s happening right across everywhere.”

Farage said the result sent a clear message that “we are now the opposition” and that if voters backed the Conservatives then they would “just get a Labour government”.

Despite the tiny margin of victory the result will heighten fears among Labour MPs that they could lose scores of seats to the hard-right populist party at the next general election. Karen Shore, Labour’s candidate, refused to speak to the media as she was ushered out of the count centre at the DCBL Stadium in Widnes, Cheshire.

The Cheshire byelection was triggered by the resignation of Labour’s Mike Amesbury, the former MP who was convicted earlier this year of punching a constituent.

Farage’s party sought to make immigration the key issue in this overwhelmingly white British corner of north-west England, raising fears over small boat crossings, houses of multiple occupancy and even Turkish barbers.

Reform UK also attacked Labour’s cutting of the winter fuel payment – an issue repeatedly raised by voters – as well as its early release of prisoners and the rising cost of energy bills.

Its tactics appeared to work, delivering Reform UK its fifth MP and establishing the fledgling party has a serious challenger to Britain’s two main parties.

In her victory speech Pochin said: “Enough is enough. Enough Tory failure. Enough Labour lies. I want to thank every one of you who were brave enough to put a cross against my name on the ballot paper.”

She said the result would “inspire the rest of the country to believe that they too can stand up for what is right and stand up for our British values”.

The result appears to back up recent opinion polls that suggested the anti-European Union populists were on course to topple Britain’s historic two-party system at the next general election.

Despite Runcorn and Helsby being one of Labour’s safest seats, the party faced a challenge to win over voters from the beginning given it was sparked by Amesbury violently assaulting a constituent in a drunken late-night row.

In the first weeks of the campaign, Shore was criticised for launching a Facebook petition to close a hotel housing asylum seekers, in what some saw as a cynical attempt to stem the flow of votes to Reform UK.

Shore, a former deputy council leader, denied her campaign was “prejudiced” but admitted “that the tone of it could’ve been slightly different – and the fact it was exploited by the populists”.

A Labour spokesperson said: “Byelections are always difficult for the party in government and the events which led to this one being called made it even harder. Voters are still rightly furious with the state of the country after 14 years of failure and clearly expect the government to move faster with the plan for change.

“While Labour has suffered an extremely narrow defeat, the shock is that the Conservative vote has collapsed. Moderate voters are clearly appalled by the talk of a Tory-Reform pact.”

Labour and Reform UK officials appeared tense as votes were counted through the night at DCBL stadium, the home of Widnes Vikings rugby league club, just across the mouth of the River Mersey from Runcorn.

Campaigners from both parties repeatedly said the result was “too close to call”, downplaying talk of a decisive victory for either side.

Turnout in the contest was a higher-than-expected 46.33%, which some on the count floor attributed to the “Farage factor” – a reference to the Reform UK leader’s ability to provoke strong opinion on either side.

There were bizarre scenes at the count centre when Reform UK officials announced that Farage was expected to arrive imminently about 30 minutes before the result was expected – a sign they were confident of victory.

But as camera crews and officials gathered, some holding the door open for their soon-to-arrive leader, there was no sign of him. Journalists were then told he was instead waiting in a car near the venue, perhaps as word reached him that it was too soon to declare a win.