The party says it is “redrawing the political map”, as it overturns huge majorities in Mid Beds and Tamworth.
The Conservatives have suffered two heavy by-election defeats, with Labour overturning huge majorities to take Mid Bedfordshire and Tamworth.
The party saw off a challenge from the Liberal Democrats to make history in Mid Bedfordshire, overcoming a 24,664 Tory majority to win the seat for the first time.
In Tamworth there was a 23.9% swing to Labour from the Tories.
Leader Sir Keir Starmer said Labour was “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,” he said.
With the Tories also trailing in the national polls, the results have provided a boost to Labour ahead of an expected general election next year.
The largely rural constituency of Mid Bedfordshire has had a Tory MP since 1931 and has never been held by Labour in its century-long history.
The 24,664 Tory majority was the biggest the party had overturned in a by-election since 1945 and Labour’s Alistair Strathern secured a swing of 20.5% to win by 1,192 votes.
In a three-way fight for the seat, the Conservative candidate Festus Akinbusoye, Bedfordshire’s Police and Crime Commissioner, came second with 12,680 votes and Lib Dem Emma Holland-Lindsay came third with 9,420 votes.
In his victory speech Mr Strathern, a former councillor who has worked for the Bank of England, said: “Tonight residents across Mid Bedfordshire have made history, after decades of being taken for granted, feeling left behind, being underrepresented, they made a decision it was time for a change.
“Nowhere is off limits for this Labour Party and tonight’s result proves it.”
Both by-elections were triggered by resignations from the previous MP, with some anger locally at the circumstances of their departure.
In Mid Bedfordshire, former Culture Secretary Nadine Dorries stood down after her name was not included on Boris Johnson’s resignation honours list.
She pledged to stand down “with immediate effect” in June, but waited more than two months before officially resigning, saying she wanted to find out why she was refused a seat in the House of Lords first.
Critics accused her of being an “absentee MP”, after not speaking in the Commons since July last year.
