The Conservatives lost two by-elections on Friday, suffering huge swings in support to the Labour party with a general election looming next year.
Keir Starmer, who at the recent party conference vowed to "heal" the UK, claimed Labour was “redrawing the political map” by taking seats which had been comfortably Conservative.
The constituencies of Tamworth and Mid Bedfordshire were up for grabs after the departure of sitting Tory MPs.
The results were announced a year to the day since his predecessor Liz Truss resigned as prime minister and leaves him with a headache as he ends his first 12 months in post.
In Tamworth, Labour’s Sarah Edwards defeated the Tories' Andrew Cooper by a majority of 1,316.
The Conservatives were defending a 19,634 majority but that was eradicated by a 23.9 percentage-point swing to Labour.
The result, announced at 2.45am, was the second-highest by-election swing towards Labour in history.
Only 30 minutes later, there was even better news for Mr Starmer as the largest majority overturned by Labour at a by-election since 1945 was announced in Mid Bedfordshire.
The Tories had held Mid Bedfordshire since 1931, with a 24,664 Conservative majority in 2019.
But Alistair Strathern took the seat with a majority of 1,192 over his Tory rival Festus Akinbusoye, with a swing of 20.5 percentage points to Labour.
Mr Starmer said: “These are phenomenal results that show Labour is back in the service of working people and redrawing the political map.
“Winning in these Tory strongholds shows that people overwhelmingly want change and they’re ready to put their faith in our changed Labour Party to deliver it.”
Mr Strathern said his victory showed “nowhere is off limits for this Labour Party”, while Ms Edwards challenged the Prime Minister to call a general election.
Both contests were triggered by the high-profile departure of their previous MPs.
Former cabinet minister Nadine Dorries quit – eventually – as Mid Bedfordshire’s MP in anger at being denied a peerage in former prime minister Boris Johnson’s resignation honours list.
In Tamworth, Chris Pincher resigned after being found to have carried out inappropriate acts at London’s exclusive Carlton Club last year – an incident which helped trigger Mr Johnson’s exit from No 10 because of his handling of the situation.
The Conservatives sought to portray the by-elections as mid-term blips, exacerbated by the difficulties surrounding the previous MPs.
But elections expert Prof Sir John Curtice said the two results were “extremely bad news” for the Conservatives and suggested Mr Sunak was on course for general election defeat.
“This isn’t destiny but it is a pointer and it is a pointer that, unless the Conservatives can fairly dramatically and fairly radically turn things around, then they are in truth staring defeat in the face in 12 months’ time.”
He warned the Tories risked seeing votes drift to Labour on the left and Reform UK on the right.
Reform secured 1,487 votes in Mid Bedfordshire and 1,373 in Tamworth, in each case more than Labour’s majority over the Conservatives.
Prof Curtice said no government had previously lost to the principal opposition party in a by-election in a seat as safe as Tamworth.
Tory former cabinet minister Robert Buckland said Mr Sunak’s party now needed to focus on the issues that mattered most to voters rather than infighting about who could succeed the party leader.
He said the Conservatives need to make it "very clear what the next five years is going to look like".
In Mid Bedfordshire, the Liberal Democrats came third and claimed their ability to switch Tory voters cleared the way for Labour’s victory.
Lib Dem deputy leader Daisy Cooper said: “We nearly doubled our share of the vote, which would see the Lib Dems win dozens of seats off the Conservatives in a general election.
“The Liberal Democrats played a crucial role in defeating the Conservatives in Mid Bedfordshire and we can play a crucial role in getting rid of this Conservative government at the next election.”