British Prime Minister Boris Johnson on Wednesday defended his government's record, vowing to fight on as he braced for a potentially damning report into lockdown-breaching parties.

The final report from an investigation by senior civil servant Sue Gray has not yet been submitted to Downing Street, the government said.

The dramatic confirmation that London's Metropolitan Police force has now started its own investigation could complicate the release of Gray's report, but opposition parties insisted on its publication in full.

Johnson, in a heated session of weekly questions in parliament, said he could not comment further on the "partygate" revelations pending the investigations.

But he said the government -- from its pandemic response to economic recovery, and "bringing the West together" against Russia's threats to Ukraine -- was not going anywhere.

"We've taken the tough decisions, we've got the big calls right, and we -- and in particular I -- are getting on with the job," the prime minister said.

Foreign Secretary Liz Truss said Gray's report was "fairly imminent", and the opposition Labour party anticipated that Johnson would give a statement in response later Wednesday or Thursday.

But regardless of its findings, Labour leader Keir Starmer said Johnson had already "shown nothing but contempt for the decency, honesty and respect that define this country", and called anew for his resignation.

Gray, described as an iron-willed enforcer of probity in government, has been investigating revelations that Downing Street staff held parties over the past two years while the rest of the country was in lockdown.

Johnson's spokesman, fending off Labour charges that a "cover-up" was in the works, said Downing Street still intended to publish the report "as received" from Gray, but could not say when.

- Snap election? -
Johnson -- the populist architect of Britain's Brexit split from the European Union -- has faced public outrage over the parties.

He attended several events, including a crowded gathering held for his birthday in June 2020 at a time when indoor socialising was banned. 

Many people have highlighted how they missed significant life events themselves in compliance with the government's rules, and were unable to comfort sick and dying loved ones struck down with Covid.

A tweet from Johnson re-emerged from March 2020 in which he told a seven-year-old girl that she was setting a "great example to us all", after she cancelled her own birthday party.

At least seven backbench Conservative MPs have called publicly for Johnson's resignation. A total of 54 letters are required to trigger a party vote of no confidence.

But cabinet ally Jacob Rees-Mogg warned wavering Tories that any successor would face strong pressure to call a snap election -- a perilous step with Labour surging to a double-digit in opinion polls.

"It is my view that we have moved, for better or worse, to essentially a presidential system," he told BBC television.

"And that therefore the mandate is personal rather than entirely party, and that any prime minister would be very well advised to seek a fresh mandate."

But along with widespread anger at Johnson, there has been no shortage of mockery on social media.

Celebrity chef Nigella Lawson ridiculed one minister's explanation that an unsuspecting Johnson had been "ambushed" with the birthday party cake by well-meaning aides.

"'Ambushed by Cake': it just has to be the title of my next book!" she tweeted.

And while Johnson touted his government's ongoing work, Conservative MP Matt Warman agreed the drip-feed of accusations was distracting.

"I definitely think it's unedifying. I definitely think it's a huge distraction from all the really important work that the government is doing and needs to get on with," he told Times Radio.