Boris Johnson was urged to explain how he funded a refurbishment of his apartment after several accusations were made against the UK prime minister by his former top adviser.
In a 1,000-word blog post, Dominic Cummings denied being the person who leaked the prime minister's phone messages and attacked Mr Johnson's handling of the coronavirus pandemic.
Mr Cummings accused his former boss of attempting an “unethical, foolish, possibly illegal” plan to get Conservative Party donors to fund a lavish refurbishment of the apartment.
On Saturday, the main opposition Labour party called for a full investigation into a series of accusations that have dogged Mr Johnson's government.
"It matters because it's about integrity and it's about taxpayers' money," Labour leader Keir Starmer told the BBC.
"You've got the former most senior adviser to the prime minister saying he's fallen well below the standards of integrity that are needed for the office of the prime minister. Every day there's more evidence of this sleaze and frankly it stinks."
Mr Johnson gave short shrift to the affair on Friday after the post by Mr Cummings.
"I don't think people give a monkey's about this issue," he told radio station LBC. "What they care about is what we were doing to protect the health of the British public and that's what I care about."
The prime minister's former chief of staff itemised a succession of alleged failings by Mr Johnson and his current senior team in the censorious blog.
“It is sad to see the PM and his office fall so far below the standards of competence and integrity the country deserves,” Mr Cummings wrote.
He called on parliament to hold an investigation into the way Mr Johnson's government responded to the pandemic and said he would be happy to give evidence under oath.
In response, Mr Johnson's office said the government followed the rules over the refurbishment. They also said Mr Johnson had never interfered in a government leak inquiry.
Mr Cummings resigned last year after a tumultuous period as Mr Johnson's chief adviser in which he clashed with officials, ripped up conventions of government and sparked a national outcry with a 400-kilometre road trip at the height of the first pandemic lockdown.
I'm not the 'chatty rat', says Cummings
The blog post followed claims in British newspapers that he had been the source of a series of so-called "chatty rat" leaks from within Mr Johnson’s inner circle last year.
"I was not directly or indirectly a/the source for the BBC/Kuenssberg story on the PM/Dyson texts,” Mr Cummings wrote, referring to BBC journalist Laura Kuenssberg.
He said that while some WhatsApp messages between Mr Johnson and UK vacuum cleaner magnate James Dyson were forwarded to him by the prime minister, the cache did not include messages leaked to the BBC this week.
The leaked texts appeared to show Mr Johnson agreeing to "fix" a tax issue for Mr Dyson concerning the treatment of overseas staff brought to the UK to work on the government's drive to mass-produce ventilators to help with the pandemic.
Mr Dyson said it was "absurd to suggest that the urgent correspondence was anything other than seeking compliance with rules” and that neither he nor his company had benefited from that project.
Mr Johnson has said he makes “absolutely no apology” for the exchange with Mr Dyson.
"If you think there's anything remotely dodgy or rum or weird or sleazy about trying to secure more ventilators at a time of a national pandemic and doing everything in your power to do that ... you're out of your mind," Mr Johnson said on Friday.
