Jeremy Corbyn, Keir Starmer.

There have been sensational by-election victories in the past. The first Liberal win in a safe Conservative seat in Orpington in 1962; the Scottish National Party breakthrough in Hamilton in 1967; the Liberal destruction of Labour in Bermondsey in 1983; George Galloway’s stealth win in Bradford West in 2012. But Labour’s win in Batley and Spen is one of the most surprising and significant.

Batley and Spen was, in effect, a Conservative seat. Labour held it in the last general election only because Paul Halloran, a pro-Brexit independent, split the Tory vote. It is similar to a swathe of constituencies that voted to leave the EU in the referendum, but where Labour held on because the Brexit Party stood, Nigel Farage deeming them not to be Tory target seats. They include Hartlepool, which the Tories won easily in the by-election less than two months ago; Ed Miliband’s seat in Doncaster; and Yvette Cooper’s seat in Normanton.

That means Labour gained ground here where it lost ground in Hartlepool. Not all northern Leave-voting seats are the same, but they are similar enough for this result to have come as a shock. Especially because Galloway, the spoiler candidate, probably took more votes from Kim Leadbeater, Labour, than from Ryan Stephenson, the Tory. It came as a surprise, too, because there had been an opinion poll in the constituency, in mid-June, which put the Tory six points ahead. If the campaign had been anything like that in Hartlepool, that should have been the end of it, because in Hartlepool support for Labour just carried on draining away every day until polling day.

This means two things. First, it is a reminder that it is hard to do constituency polls. This one was based on a sample of 510, just 323 of whom gave a voting intention, so the margin of error was easily enough to wipe out that Tory lead. Second, it means that the Batley and Spen campaign was very different from that in Hartlepool. Kim Leadbeater was a stronger candidate than Paul Williams: energetic, outgoing and resolutely local. The Labour organisation, undistracted by local elections, was better. And Galloway’s intervention gave the campaign a story.

The last few weeks were dominated by the clash between Galloway and Leadbeater, summed up by the visual image of Leadbeater being shouted at in the street by Galloway supporters but appearing to stand up for herself. That does seem to have galvanised Labour support and shifted the story from one of Labour passivity and neglect.

The immediate effect is to end the speculation about Keir Starmer’s position as Labour leader. Galloway stood on a “Starmer Out” platform, and Labour won. Labour was able to withstand his divisive politics, and those Corbyn supporters who wanted Galloway to succeed have now been comprehensively defeated. Batley and Spen really is the end of the road for Corbynism.

Starmer has shown that he can turn the tide in the red wall: that he can, in effect, gain seats from the Conservatives.

Just as importantly, Labour’s win in Batley and Spen changes the Boris Johnson story. The miracle vote-getter of Hartlepool has been brought down to earth by the normal laws of politics, which is that governments don’t gain seats in by-elections. The cynical pre-election timing of the prime minister’s visit to the Nissan plant in Sunderland, to coincide with the company’s announcement that it would be creating 1,600 jobs, failed to work its magic on the voters of West Yorkshire.

That means that Starmer, who put Boris Johnson on the defensive at Prime Minister’s Questions this week, will continue to press his advantage. With Labour starting to close the gap in national opinion polls, it will be Conservative MPs’ turn to feel nervous.