Johnson's opponents say he may have broken British rules by letting party donors secretly contribute tens of thousands of pounds to a luxury renovation of his living quarters.
In a hot-tempered appearance in Parliament on Wednesday, Johnson repeatedly insisted he had covered the cost of the renovation himself, though he avoided directly answering whether he had done so only after the fact, once the scandal had emerged.
Though Johnson has repeatedly weathered opprobrium for gaffes, crises over Brexit and disclosures about his adultery, he is now grappling with an array of accusations which opponents say show that he is unfit for office.
"We are now satisfied that there are reasonable grounds to suspect that an offence or offences may have occurred," the Electoral Commission said of the financing of the apartment above Number 11 Downing Street, where Johnson resides with his fiancee and 11-month old son.
"We will therefore continue this work as a formal investigation to establish whether this is the case," the commission said.
If it finds sufficient evidence of an offence, the commission can issue a fine of up to £20,000 ($NZ38,000) or refer the matter to the police.
FORTUNES RECOVERING
After muddling his initial response to the coronavirus outbreak, Johnson has seen his political fortunes recover this year as his government organised one of the fastest vaccine rollouts in the world. The economy is set to rebound sharply after contracting 10% in 2020.
But disclosures that Johnson and his 33-year-old fiancee, Carrie Symonds, spent lavishly to redecorate their residence with a designer feted by royalty have touched a nerve.
Just days before May 6 local elections across the United Kingdom, Johnson, at times shouting in exasperation and red in the face, said he had covered the costs and followed the rules.
"The answer is I have covered the costs," said Johnson under questioning in parliament from opposition Labour Party leader Sir Keir Starmer, who cast Johnson as "Major Sleaze".
Starmer said Johnson had been selecting wallpaper for £840 ($NZ1600) a roll in the middle of the pandemic, and reminded Johnson that ministers who knowingly misled Parliament are expected to resign.
Johnson's supporters deny he has done anything wrong and say he is focused on the Covid-19 crisis.
WHO PAID?
Johnson has a taxpayer-funded £30,000 allowance each year for maintaining and furnishing his official residence. Costs above that must be met by the prime minister himself.
Johnson and his ministers have repeatedly said he paid for the work. But they have not said when he paid, or whether the refurbishment, reported to have cost £200,000, was initially financed by a loan which he would have been required to report under political financing rules.
Asked last month about the refurbishment, Johnson's spokeswoman said all donations and gifts were properly declared, and no Conservative Party funds were used to pay for it.
Johnson was asked in Parliament if the refurbishment was partially financed by a £58,000 donation from Conservative Party donor David Brownlow.
"The answer is that I have covered the cost," Johnson replied.
Dominic Cummings, who was Johnson's main adviser on the Brexit campaign and helped him to win an election in 2019 before an acrimonious split last year, said on Friday that Johnson had wanted donors to pay for the renovation secretly.
Cummings had told the prime minister such plans were "unethical, foolish, possibly illegal", he said.
In a further potentially damaging allegation, the Daily Mail newspaper on Sunday cited unidentified sources as saying that, in October, shortly after agreeing to a second lockdown, Johnson had told a meeting in Downing Street: "No more f****** lockdowns - let the bodies pile high in their thousands."
Asked in Parliament if he had used those works, Johnson repeatedly denied that he had.
"No," Johnson said. "I didn't say those words."
