Becky Morton
Political reporter
Getty Images
Reform has won the Runcorn and Helsby by-election by just six votes, beating Labour after a dramatic recount.
In Sir Keir Starmer's first major electoral test as prime minister, former Conservative councillor Sarah Pochin overturned a 14,696 majority to take the Cheshire seat for Reform.
The by-election was triggered by the resignation of former Labour MP Mike Amesbury, following his conviction for assaulting a constituent.
As well as winning its first ever parliamentary by-election, Reform also took its first mayoralty in Greater Lincolnshire - though Labour held on in three other mayoral races.
It means Reform UK - originally called the Brexit Party when it was founded in 2018 – now has five MPs.
Labour requested a recount, after party officials told the BBC the first tally put Nigel Farage's party ahead by just four votes.
The final result beats the previous post-war record for the closest by-election, which was won by just 57 votes in Berwick-upon-Tweed in 1973.
Farage, who appeared at the count shortly before the result was declared, said it "proves we are now the opposition party to this Labour government".
"With this and other results tonight, it's clear that if you vote Conservative you will get Labour," he wrote on X.
"But if you vote Reform, you get Reform."
In her victory speech, Pochin - Reform's first female MP - said: "The people of Runcorn and Helsby have spoken - enough is enough.
"Enough Tory failure, enough Labour lies."
A Labour spokesperson said: "By-elections are always difficult for the party in government and the events which led to this one being called made it even harder."
They added that "voters are still rightly furious with the state of country" after 14 years of Conservative rule and "expect the government to move faster".
But they argued the Tory vote had "collapsed", at just 2,341.
One Labour campaigner told the BBC the government's controversial decisions to cut winter fuel payments for pensioners and disability benefits had affected the result.
"On every door it was the same story - winter fuel and PIP [personal independence payment]," they said.
A Conservative Party spokesman said Sir Keir's political programme had been "roundly rejected" and Labour MPs would "rightfully question his leadership" following the result.
Earlier, candidates at the count gathered with election officials as bundles of ballot papers were checked.
One party member was seen holding up four fingers to a colleague, before a recount was confirmed.
Reuters
Reform UK chairman Zia Yusuf (left) celebrated alongside Farage at the count
In Greater Lincolnshire, former Conservative MP and minister Andrea Jenkyns, who defected to Reform last year, became the region's first mayor.
She won with a majority of almost 40,000 over the Conservatives.
In early results, Labour held the North Tyneside and Doncaster mayoralties by just a few hundred votes.
It also held on in the West of England mayoral contest, with a majority of 5,949, as the Tories were pushed to fourth place behind Reform and the Greens.
Labour saw its share of the vote fall significantly as Reform came a close second in all three races.
Reform is still hoping to make further gains in council elections, where most counting is starting later.
Reform's deputy leader Richard Tice told the BBC he believed the party's candidate in the Greater Lincolnshire mayoral race, Andrea Jenkyns, had won "by a considerable margin".
The party is also targeting councils including Kent and Lincolnshire, where the Conservatives currently have large majorities, while it is aiming to make gains at the expense of Labour on councils like Doncaster.
In the Hull & East Yorkshire mayoral contest, it could be a closely fought race between the Tories, Labour, the Lib Dems and Reform's candidate, former Olympic boxer Luke Campbell.
Support for Reform UK has been rising in national polls since last year's general election, when the party secured more than four million votes, coming third behind Labour and the Conservatives.
You can follow all the results and reaction on BBC One and on iPlayer, the News Channel, Sounds and on the BBC live page.