It has been a stonking night for Labour – winning two more by-elections in safe Conservative seats on substantial swings.

The momentum of last year’s gains in Tamworth, Selby and Ainsty, and Mid Bedfordshire, which some in the party had feared would stall, has continued to accelerate in the first tests of the general election year.

Tories crash to new postwar low with double defeat – live updates

Of course, by-elections can deliver fireworks that do not directly translate into general election results, but these polls appear dramatic and consequential.

First, because they show British politics has turned on its head in just three years.

Here is a number to show the scale of the tumult – in May 2021, less than three years ago, a seemingly unassailable Boris Johnson took his party into a by-election in Hartlepool and won the seat from Labour with the biggest increase in vote share to a governing party since 1945.

Tories expected defeat – but not on this scale

Overnight in Wellingborough, a safe Tory seat with an 18,500 majority barely on Labour’s campaign radar until a few months ago, was won on a swing of 28.5%.

The second-biggest swing since 1945 and the largest-ever drop in Tory vote share.

It had been at number 226 on Labour’s target list.

Senior Conservatives I spoke to in the final 24 hours of campaigning had predicted a double defeat, but not one on this scale.

They had hoped to hold on to 60% of their vote and that the swing would be less dramatic than in Tamworth and Selby at almost 24%.

In fact, in Wellingborough it far exceeded that swing, and vote share collapsed.

Uniquely cursed contests – or potentially terminal?

The picture was better in Kingswood, but the worry in Conservative HQ is that however you cut it, this looks potentially terminal.

The Conservatives will say today these contests were uniquely cursed.

In Wellingborough, they had a soap opera with the long-serving MP Peter Bone, who was found to have bullied and exposed himself to a member of staff, allegations he denies. His partner Helen Harrison was running in his place; leading the national party to keep its distance from this campaign.

In Kingswood, the MP Chris Skidmore who was standing down anyway, triggered a by-election that Tories didn’t need over net zero policy – in a seat which is being abolished in boundary changes.

Tory MPs did not hold back on their anger about it – and voters were telling Labour campaigners they didn’t understand “what the bloody point of this is”, as one MP reported back.

Even with that, Labour overturned an 11,000 majority with a swing of 16 points.

Threats from the left and the right

Sir Keir Starmer, jubilant after a difficult couple of weeks, which still raise ongoing questions about his judgement, said the contests show people “want change” and “Conservative voters are switching directly to this changed Labour Party”.

There is another subplot to this by-election which is the rise of the Reform Party, which achieved its best results at 10% in Kingswood and 13% in Wellingborough.

In the former, that’s enough votes that if they had gone to the Conservatives instead, they would have claimed victory.

Jacob Rees-Mogg was quick to tell Sky News, it is time to “unite the Conservative family” by bringing them into the fold.

Reform’s impact should not be overstated – the party’s forerunner UKIP used to win by-elections outright; and both Labour and the Conservative accuse the party of underperforming in this contest.

But leader Richard Tice has vowed not to do any deals to unite the right-wing vote and that presents a challenge.

The Conservatives were worried – putting out literature in Wellingborough, seen by Sky News, specifically targeting Reform voters.

And they may not want to help Rishi Sunak, with polling by YouGov and others suggesting far from all of Reform voters will return to the Conservatives at general election time.

Bad sign for general election

All this presents the prime minister with a terrible backdrop for the final few months before the election – with few opportunities to change the narrative, although the forthcoming Budget is one.

More of his internal critics could raise their heads above the parapet to call for a shift in strategy – or even leadership – after weeks of grumbling by critics of his Rwanda policy.

The parties will pick over the numbers in the coming hours and days, in particular on the question of how many voters were direct switchers from the Tories to Labour.

But as the choice in the general election looms, it’s another hammering which confirms Labour’s momentum.

And one which exposes another difficult front for the Conservatives on their right flank. Conservative MPs are unlikely to write it off as a bad day at the office.