The local election results will be seen as a barometer of support for Johnson's Conservatives nationally, as well as an indicator of whether the opposition Labour party poses a serious threat.
Johnson, 57, won a landslide general election victory in December 2019 on a promise to break years of political deadlock and deliver Brexit the country's divisive departure from the European Union.
But his position has looked increasingly fragile, because of damaging claims about lockdownbreaking parties at Downing Street and an inflationary surge that is squeezing voters' incomes.
A police investigation last month saw him become the first British prime minister to be fined for breaking the law while in office.
Irate Tory MPs, mindful of public outrage at double standards and denials, had looked set to force a noconfidence vote in his leadership in January.
But Russia's February invasion of Ukraine, during which Johnson has shown hawkish support for President Volodymyr Zelensky, took the heat out of any mutiny.
Cost of living
A drubbing for Johnson's Tories on Thursday, though, could revive calls for him to go, to bed in a new leader for the next general election, which is due by 2024.
"Partygate", however, has not proved the key issue for voters.
"What's going to get folks a lot is the cost of living: food is going up, energy is going up," said one voter, who gave his name only as Bob, in Dudley, central England.
"What he (Johnson) did was bad, with partygate, they were more or less laughing at you," the 76yearold retired factory worker told AFP.
"But they should focus on cost of living."
Labour the main opposition nationally gained ground at the local level in 2018, with the Tories in disarray after the Brexit vote two years earlier.
Keir Starmer, leader since 2020, will be hoping to claw back power on councils in "Red Wall" Labour areas of England that turned Tory blue at the last general election.
Polling indicates Labour will win the most seats in England, while the party wants to gain ground on the proindependence Scottish National Party (SNP) in Scotland and consolidate its hold on Wales.
Union issue
Apart from Johnson, the longterm future of the United Kingdom of Great Britain and Northern Ireland may also be in jeopardy this week.
Elections are also being held for the powersharing assembly in Belfast, with Sinn Fein widely tipped to become the biggest party.
The Belfast Telegraph's LucidTalk poll on Friday found that nationalists are six points ahead of their closest rival, the Democratic United Party (DUP).
No pro-Irish nationalist party has become the largest party in the turbulent century of British provincial history.
Deirdre Heenan, professor of social policy at the University of Ulster, called it "a watershed in Irish politics."
"If a nationalist becomes the first minister, it will be a dramatic change," she told AFP.
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