Labour appear increasingly confident of overturning huge Conservative majorities in the Mid Bedfordshire and Tamworth by-elections.
Shadow cabinet minister Peter Kyle said “clearly we’ve done very well” in the contests in what had been safe Tory seats.
The Conservatives said holding the two constituencies would be “challenging” for any incumbent government.
But the Tories have held Mid Bedfordshire since 1931 and the 24,664 Conservative majority would be the largest overturned by Labour at a by-election since 1945.
In Tamworth, Rishi Sunak’s party is defending a 19,600 majority and it would require a dramatic 21.4 percentage point swing for Labour to claim it.
Defeat in both would deal a significant blow to the Prime Minister, with a general election expected next year.
Both seats are vacant after 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 Mr Johnson’s exit from No 10 because of his handling of the situation.
A Conservative Party spokesman said: “These were always going to be challenging by-elections and the rule of thumb is that governments don’t win them.
“We have seen little to no enthusiasm for Sir Keir Starmer who voters can see stands for nothing and always puts short-term political gain first.
“It is clear that we have to maintain our focus on people’s immediate priorities whilst taking long-term decisions which will lead to the change this country needs.”
Shadow cabinet minister Peter Kyle, who has managed Labour’s campaign in Mid Bedfordshire, told Sky News: “Clearly we’ve done very well in this by-election.
“Clearly we’re doing very well in the Tamworth by-election as well.”
Shadow media minister Stephanie Peacock, who spearheaded the Tamworth campaign, told the PA news agency: “It’s certainly looking like an encouraging night for Labour. The votes are still being counted but it’s certainly looking positive.
“It’s a really big majority, so the fact that Labour are even on the pitch, even in contention, in this election is quite astonishing.”
In Tamworth, Labour’s Sarah Edwards hopes to defeat Tory Andrew Cooper.
Ms Peacock said while defeat in Tamworth would not necessarily mean the party could not win the next general election, victory would be a “stunning result” for Labour.
She said: “We do not need to win Tamworth to win a general election. The road to a Labour government in Number 10 runs through the West Midlands, of course, but it doesn’t necessarily run through Tamworth.
“We could lose this seat in the general election and still form a government, but if we managed to win, it doesn’t matter how close, a majority of one is a pretty amazing result for the Labour Party.”
In Mid Bedfordshire, a three-way fight has seen the Liberal Democrats taking on Labour and the Tories, with Conservative candidate Festus Akinbusoye hoping to benefit from a split in an anti-government protest vote.
Labour’s Alistair Strathern and the Lib Dems’ Emma Holland-Lindsay have both received high-profile backing from their party leaders in a contest which they have had plenty of time to prepare for – Ms Dorries originally said she would quit in June.
The Lib Dems suggested they had succeeded in winning over Tory voters, clearing the way for a Labour victory in Mid Bedfordshire.
A Lib Dem source said: “We think we’ve doubled our share of the vote, and taken enough Conservative votes in the villages to help Labour get over the line.”
