Polls closed on Thursday in two crucial byelections that threaten to pile more pressure on beleaguered Prime Minister , who vowed not to resign even if his ruling lose the highly symbolic seats.
The Tories are tipped to lose the contest for the parliamentary seat of Tiverton and Honiton in southwest England – a true blue heartland which has voted Conservative in every general election since the 1880s.
They are also expected to lose Wakefield in the north, part of the so-called Red Wall which switched from Labour to Tory in Johnson's 2019 landslide.
The votes are being held after the constituencies' former Tory MPs both resigned in disgrace.
Tiverton and Honiton's ex-lawmaker Neil Parish quit after admitting watching pornography on his phone in the House of Commons, while Wakefield's Imran Ahmad Khan was jailed for sexually assaulting a teenage boy.
The byelections also follow months of scandals and setbacks that have severely dented the popularity of Johnson and his party, and come just weeks after he narrowly survived an attempt by his own lawmakers to oust him as Tory leader and prime minister.
The 6 June vote among Conservative MPs saw more than 40 percent of the parliamentary party desert Johnson, leaving him severely weakened and struggling to reset his turbulent tenure.
As voters head to the polls the embattled UK leader is thousands of miles away attending a Commonwealth summit, from where he again insisted quitting was not on his mind.
"Are you crazy?" he replied to reporters travelling with him when asked if twin defeats on Thursday could trigger his resignation.
"Governing parties generally do not win byelections, particularly not in mid-term," Johnson added.
"I'm focused entirely on delivering on the agenda of this government."
Johnson has spent months fighting for his survival after a series of controversies including the "Partygate" saga led many Tories to question whether he should remain as leader.
Various opinion polls have shown the public think he lied about Covid lockdown-breaking events at Downing Street and should resign.
Even before the controversy erupted last December, the 58-year-old Brexit architect saw the loss of two once-safe seats in byelections last year.
He then scored dismally in May's local elections.
The latest potentially perilous polls opened on Thursday at 7am (0600 GMT) and closed at 10pm, with results expected in the early hours of Friday.
The Conservatives won Tiverton and Honiton in 2019 by more than 24,000 votes but the constituency looks set to switch hands following what its ex-MP Parish described as an indefensible moment of "total madness" watching porn in parliament.
The small opposition Liberal Democrats are hopeful of success, with party leader Ed Davey saying they were "neck and neck".
Residents are "fed up of Boris Johnson's lies and neglect," he added.
Wakefield, near Leeds, was one of dozens of former Labour seats that Johnson took in 2019 on a promise to "get Brexit done" and address glaring regional economic inequalities.
But it could now flip back due in part to Johnson's waning popularity.
Retired teacher Judy Froggat said local people wanted to give Johnson "a bloody nose".
"I'm quite incensed by a lot of the things that he's done," she told AFP after casting her ballot, branding the prime minister "a proven liar".
The polls come with Britain gripped by 40-year highs in inflation and a cost-of-living crisis that has seen prices soar for everyday essential such as energy, petrol and food.
Strikes this week by railway workers – some of the biggest seen in Britain in decades – have added to the sense of crisis.
But the Wakefield contest also comes with risks for Labour, which needs to secure such seats if it is to win the next general election due by 2024.
Anything less than a convincing win in Wakefield is likely to be seized upon by critics of Labour leader Keir Starmer as further evidence of his inability to complete the rebuild and return the party to power after 12 years in opposition.
Retired Wakefield resident Moretta Pullan, 79, said people were "disillusioned" with both of Britain's main political parties, but was bearing the brunt of that.
"So I think people are [at a] crossroads, really, as to who to vote for," she told AFP.
"We all put our hands up for Boris and thought he was going to be the most wonderful leader we've ever had and lead us to the glory land, and he hasn't."