UK Prime Minister Boris Johnson Wednesday offered "heartfelt apologies" for attending a lockdown-breaching party held in his Downing Street garden, but deflected calls to resign as the opposition leader called him a "man without shame".

Breaking his silence over the latest of a slew of allegations regarding top-level misbehaviour, Johnson said he regarded the boozy get-together in May 2020 as a work event for Downing Street staff.

He added that he did not appreciate how it would look to millions of Britons who were respecting Covid rules, even missing out on farewells to dying relatives.

"And to them and to this House I offer my heartfelt apologies," Johnson told a stormy session of questions in the House of Commons.

Keir Starmer, leader of the main opposition Labour party, dismissed the apology as "worthless" and mocked Johnson for belatedly speaking out after "months of deceit and deception". 

"Is he now going to do the decent thing and resign?" Starmer said, demanding the Conservative leader's head for the first time and arguing: "The prime minister's a man without shame." 

Even some on his own side want Johnson to go, but in response to Starmer, he urged all sides to await the findings of an internal inquiry he has commissioned by a senior civil servant.

There has been a flurry of accusations about Downing Street parties held during lockdowns in 2020 which have dogged Johnson since late last year, sparking widespread public anger and sinking poll ratings.

Lisa Wilkie was forced to film her brother dying of Covid in intensive care in May 2020, because her mother was not allowed to visit the hospital under the restrictions.

"People died sticking to the rules, and they broke those rules to have a bottle of wine," a tearful Wilkie told the BBC.

Do the right thing

The prime minister had stonewalled the issue since an email was leaked late Monday in which a senior aide invited more than 100 colleagues to the event on May 20, 2020, encouraging them to "bring your own booze".

Both Johnson and his wife Carrie attended the gathering, according to anonymous witnesses quoted in the media, intensifying anger even among party colleagues.

"If the prime minister knowingly attended a party I don't see how he can survive, having accepted resignations for far less," Conservative lawmaker Nigel Mills told the BBC earlier.

The event occurred when the government was ordering members of the public not to meet, even outdoors, and tight restrictions were in place on social mixing, including at funerals. 

Police at the time fined those breaching the rules, and had the option to prosecute repeat or egregious offenders. 

Hannah Brady, whose father's death certificate was being signed on May 20, 2020, penned an open letter with other bereaved families Johnson has personally met, urging him to "do the right thing" and explain what happened.

Lying low 

Even the front pages of newspapers that normally back Johnson and the Tories were damning.

"Is the party over for PM?" asked the best-selling Daily Mail, while the Daily Telegraph's headline said: "Johnson losing Tory support."

"It's my party and I'll lie low if I want to," mocked The Sun tabloid.

Johnson had hoped to start the new year afresh, leaving behind the accusations of lockdown-breaking parties and separate claims of cronyism and corruption that contributed to a shock by-election defeat for the Tories before Christmas.

Meanwhile, London's Metropolitan Police have said they have been in contact with the Cabinet Office about the May 2020 gathering, raising the possibility of a more serious, criminal probe.

The ongoing furore appeared to be proving increasingly unbearable for Conservative lawmakers. 

"How do you defend the indefensible? You can't!" Tory MP Christian Wakeford tweeted Wednesday.

"It's embarrassing and what's worse is it further erodes trust in politics when it's already low."