The polls will be voters' first chance to give their verdict on Johnson since a damning government report last month detailed a series of illegal lockdown-breaking parties at his Downing Street office during the COVID-19 pandemic.

Earlier this month, he survived a no-confidence vote by lawmakers from his Conservative Party, but his premiership remains mired in a series of scandals and he is under investigation by a parliamentary committee for misleading parliament.

But Johnson, speaking to reporters on the way to Rwanda for a Commonwealth meeting, rejected any suggestion he could be forced out by defeat in the two by-elections.

"Are you crazy?" he said when asked if he would resign if he lost both seats.

"Governing parties generally do not win by-elections, particularly not mid-term. You know, I am very hopeful, but you know, there you go. That's just reality."

The two seats, one in northern England and the other in the south, are up for election after the Conservative incumbents resigned in disgrace - one for watching porn in parliament and the other after he was found guilty of sexually abusing a teenage boy.

If the Conservatives lose one or both elections, it is likely to lead to renewed questions about Johnson's leadership despite him surviving the no-confidence vote.

Although under his party's rules he cannot be challenged for another year, a defeat could lead lawmakers fearing for their own futures to reduce the grace period to bring about another confidence vote.

When Johnson was asked if he was concerned his lawmakers could change the rules to challenge him within a year, he said:

"I'm focused entirely on delivering on the agenda of this government. My golden rule is the less (you) talk about Westminster issues, the more you talk about the things you want to talk to the country about."