Labour has “made history”, Sir Keir Starmer said, after his party dealt a double by-election blow to Rishi Sunak by overturning huge Tory majorities in Tamworth and Mid Bedfordshire.

The party leader vowed to “repay” people who lent Labour their vote in the two seats, which both previously had comfortable Conservative majorities.

Appearing alongside winning candidate Alistair Strathern at the Forest Centre & Millennium Country Park near Bedford, Sir Keir said: “What a fantastic candidate… He has not only won here, he’s made history here over the past 24 hours. An incredible achievement already.”

Sir Keir added: “We know that voters here have voted for us and they’ve put their trust and their confidence in a changed Labour Party, and we will repay them for that trust and confidence.

“We do so humbly. And I know there are people yesterday who probably voted Tory in the past who voted for a changed Labour party because they despair at the state of their own party.

“I’m glad that they see that our party is changed and they can put their trust and confidence in them, and I hope that that persuades other voters across the country who may have voted for other parties in the past, that the party of the future, the party of national renewal to reject the decline of the last 13 years, is this changed Labour Party.”

Sir Keir claimed Labour was “redrawing the political map” by taking seats which had been comfortably Conservative, ahead of the general election expected next year.

In Tamworth, Labour’s Sarah Edwards defeated Tory Andrew Cooper by a majority of 1,316.

The Conservatives were defending a 19,634 majority, but a 23.9 percentage point swing to Labour saw that eradicated.

The result, announced shortly at 2.45am, was the second-highest ever by-election swing to Labour.

Just half an hour later, there was even better news for Sir Keir as Mid Bedfordshire saw the largest majority overturned by Labour at a by-election since 1945.

The Tories had held Mid Bedfordshire since 1931, with a 24,664 Conservative majority in 2019.

But Mr 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 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.

“My message to the Prime Minister is: get in your Government car, drive to Buckingham Palace, do the decent thing, and call a general election,” she said.

Both contests were triggered by the high-profile departures of their previous MPs.

Former cabinet minister Nadine Dorries quit – eventually – as Mid Bedfordshire’s MP in anger at being denied a peerage in Boris Johnson’s resignation honours list.

In Tamworth, Chris Pincher resigned after being found to have drunkenly groped two men in an “egregious case of sexual misconduct” at London’s exclusive Carlton Club last year – an incident which helped trigger Boris 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 Professor 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.

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 both instances more than Labour’s majority over the Conservatives.

Prof Curtice told the BBC: “No government has hitherto lost to the principal opposition party in a by-election a seat as safe as Tamworth.”

He added that the Tories “may get caught in a pincer movement between some of their former Leave voters wandering off to Labour but others going off to Reform UK”.

The Tamworth result echoes Labour’s victory in a by-election in its predecessor constituency South East Staffordshire in 1996.

The Conservatives went into that contest defending a large majority only to see Labour win the seat on a swing of 22.1 percentage points before a general election landslide the following year – a result Sir Keir would dearly like to repeat.

On Friday’s morning media round, Conservative Party chairman Greg Hands sought to blame the “legacy issues” predating Mr Sunak’s premiership which led up to the two by-elections, suggesting people “were happy with the job Rishi Sunak is doing as Prime Minister”.

He pointed to low turnout in both votes, with 44% in Mid Bedfordshire – down from 74% at the last general election – and 36% in Tamworth, down by around 28%.

“I don’t see any enthusiasm for Labour, but clearly there’s been a lot of, if you like, background circumstances in those two by-elections that have also made the job difficult for us,” he told Times Radio.

Mr Sunak was out of the country as the by-election results came in, spending the night in Saudi Arabia on a tour of the Middle East in the aftermath of the Hamas attacks on Israel.

The results were announced a year to the day after Mr Sunak’s predecessor Liz Truss resigned as prime minister and leave him with a headache as he ends his first 12 months in post.

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.”