Boris Johnson offered the House of Commons his "wholehearted apology" on Tuesday after being fined by the Metropolitan Police for participating in a Downing Street party during the coronavirus lockdown.
According to Johnson, the Britons "had a right to expect better of their prime minister". He reiterated that he believed he had done nothing wrong at the time.
"That was my mistake and I apologise for it unreservedly," he said.
Labour leader Sir Keir Starmer, when speaking to Parliament, called Johnson "a man without shame", urging him to resign as he weathers the "partygate" storm.
On Thursday, MPs will decide whether to pursue an investigation against Johnson based on the claims that he misled Parliament over the "partygate" row. If Johnson is found to have done so, he would likely be expected to step down.
The prime minister argued he would not be resigning, citing the ongoing crisis in Ukraine and the spike in cost-of-living triggered by it.
Last week, the Metropolitan Police slapped a fine on Johnson for participating in his surprise birthday bash in Downing Street during the lockdown. By receiving the penalty, Johnson essentially became the first-ever British PM to be found to have broken the law while in office.
With the investigation into the "partygate" row continuing, many observers expect the prime minister to face even larger fines, given that the birthday bash is not the only lockdown party he and his government's officials are suspected of attending. Among such events were alleged "bring your own booze" parties and wine gatherings in Downing Street, along with a Christmas party and several leaving-dos.