The prime minister is expected to say he did not knowingly break the rules at a 2020 birthday party at No 10.
Opposition parties have accused him of lying to Parliament after he previously told them no rules had been broken.
Mr Johnson has vowed to “set the record straight” when he speaks to MPs at about 3.30pm.
But he is also expected to urge MPs to focus instead on issues such as rising prices and the war in Ukraine.
Speaking to ITV, Labour leader Sir Keir Starmer accused the PM of using the conflict in Ukraine as a “shield” to keep his job, adding that he finds such a tactic “pretty offensive”.
Sir Keir predicted: “He will try an try an apology, and he will immediately then go into excuses”.
Labour and the other opposition parties are exploring ways to hold the prime minister to account, such as a censure motion, effectively a slap on the wrist, or a vote of no confidence.
But shadow attorney general Emily Thornberry said a coup by Tory MPs was needed to remove the prime minister.
She told BBC Radio 4’s Today programme: “Whatever means we take, the difficulty we will always have is that, since the 2019 election, the Conservatives have an 80-seat majority when there is a vote.
“Unless Conservative MPs can look at their consciences and vote the right way, we are not going to get the sort of result that we should get.”
Commons Speaker Sir Lindsay Hoyle is expected to decide around lunchtime what vote – if any – should take place on measures against the prime minister.
Mr Johnson became the first serving UK prime minister to be sanctioned for breaking the law when, along with his wife Carrie and Chancellor Rishi Sunak, he was fined last week for attending the birthday event held for him in the Cabinet Room in June 2020.
The PM is facing accusations of lying to MPs because he initially told them Covid rules had been followed in No 10 after the first reports of parties emerged last year.
Under government rules, ministers are expected to resign for knowingly misleading MPs – and correct the record as soon as possible if they inadvertently tell Parliament something false.
Mr Johnson has since said it “did not occur” to him at the time of the gathering that it could be in contravention of Covid lockdown rules.
On Monday, Transport Secretary Grant Shapps said what happened was “wrong”, but argued Mr Johnson had not “knowingly” misled MPs.
Opposition parties strongly believe the prime minister deliberately misled parliament and are looking at various mechanisms to hold him to account.
Labour sources say options under consideration include forcing a vote on whether he misled Parliament, or referring him to a parliamentary committee to investigate.
There’s no suggestion the prime minister would lose any vote against him, but opposition parties are likely to strongly criticise any Tory MPs who may be forced to publicly defend his actions.
After fresh allegations in the Sunday Times that at a leaving drinks for his director of communications, Lee Cain, in November 2020 the prime minister poured drinks at the gathering Labour’s deputy leader Angela Rayner said – if the reports were true – then the prime minister had a “hand in instigating” at least one of the parties.
No 10 have not denied that this event took place, but have strongly denied that the prime minister instigated it.
However, there is concern among some Tory MPs that further fines or bad headlines could be yet to come.
Alongside staff leaving parties on 18 June 2020 and 16 April 2021, the June 2020 birthday event is among three so far known to have attracted fines.
Mr Johnson is known to have attended at least two further events of the 12 being investigated by police, meaning he could be fined again.
More than 70 Conservative MPs have expressed their support for the PM since he was fined, including some who have previously called for him to go.
Since Mr Johnson was fined last week, only a handful of Tory MPs have publicly said he should quit.
What has the PM told MPs about parties?
When asked whether there was a party in Downing Street on 18 December 2020, the PM told the Commons on 1 December 2021 that “all guidance was followed completely in No 10”.
After the publication of a video showing No 10 staff joking about the 18 December event, he told MPs on 8 December 2021 he had been “repeatedly assured” that “there was no party and that no Covid rules were broken”.
Later that day, he told the Commons he was “sure that whatever happened, the guidance was followed and the rules were followed at all times.”
On 12 January 2022, he apologised for attending a Downing Street garden party on 20 May 2020 but said he had “believed implicitly” it was a work event.
