Labour has overturned two large Tory majorities in the biggest indication yet that Sir Keir Starmer will lead the next Westminster government.

Safe Conservative seats in Mid Bedfordshire and Tamworth were snatched by Labour whose leader said the party was “redrawing the political map” ahead of a general election expected next year.

In Mid Bedfordshire, Alistair Strathern was returned with the largest majority overturned by Labour at a by-election since 1945. The flipping of nearly 25,000 votes breaks the record for the largest numerical majority overturned at a by-election.

Sarah Edwards overturned a 19,000 Conservative majority in Tamworth, a seat the party won from the Tories in a by-election in 1996 before winning a landslide at the general election.

The result in Tamworth was the second-highest-ever by-election swing to Labour. Edwards used her acceptance speech to demand Prime Minister Rishi Sunak call a general election.

Sir Keir said: “Winning in these Tory strongholds shows that people overwhelmingly want change, and they’re ready to put their faith into our changed Labour Party to deliver it.”

Both of these seats were won comfortably by the Conservatives at the last election. A Tory spokesman said there were specific reasons for them being lost last night.

Nadine Dorries resigned from Mid Beds after being denied a peerage in Boris Johnson’s resignation honours, while Chris Pincher stood down in Tamworth following allegations around his behaviour in London’s Carlton Club.

It is the first time Labour have made two by-election gains on the same day since 1962 and the two results cap a run of four wins for the party after it won in Selby and took the Rutherglen and Hamilton West seat with a higher-than-expected 20.4 percentage point swing from the SNP.