Dominic Cummings claimed the PM had once had "possibly illegal" plans to get Tory donors to fund the work.

The government said "costs of wider refurbishment in this year have been met by the prime minister personally".

But Labour said Mr Johnson needed to fully disclose where the money came from and whether it was a loan.

Party leader Sir Keir Starmer said: "It matters. It is about integrity, it is about taxpayers' money. Every day there is more evidence of this sleaze. Frankly, it stinks.

"Publish the details, have the full inquiry. If there is nothing to see here - whether it is the refurb of No 10, whether it is the dodgy contracts, whether it is the privileged access - if there is nothing to see, publish everything, have a full inquiry.

"Sunlight is the best disinfectant," Sir Keir said.

It comes amid a row over the lobbying of ministers, including the prime minister, after it was revealed he had exchanged text messages with businessman Sir James Dyson. Mr Cummings denied reports he leaked the messages.

Former attorney general Dominic Grieve said the "constant wriggling" over payment for the flat refurbishment was "just one illustration of the chaos Mr Johnson seems to bring in his wake", describing the prime minister as "a vacuum of integrity".

The former Conservative MP, who was expelled from the parliamentary party by Mr Johnson in 2019 after rebelling against him in a Brexit vote, also described his government as "a cronyistic cabal".

Once Mr Johnson's closest ally, Mr Cummings was forced out of his Downing Street role at the end of last year, following an internal power struggle.

In a blistering attack on his old boss on Friday, Mr Cummings questioned the prime minister's "competence and integrity".

In his first blog post since leaving his role, Mr Cummings:

*  denied leaking text messages sent between Mr Johnson and Sir James

*  denied leaking details of the second coronavirus lockdown in England in November

*  claimed the prime minister once had a "possibly illegal" plan for donors to pay for renovations of his Downing Street flat

*  alleged Mr Johnson had considered trying to block an inquiry into the leak in case it involved a friend of his fiancee Carrie Symonds

No 10 insisted the PM had never interfered in a government leak inquiry.

Mr Cummings promised to answer questions about "any" issues when he appears before a Parliamentary inquiry into the government's pandemic response on 26 May.

Sir Alistair Graham, former chairman of the Committee on Standards in Public Life, said if Mr Cummings could give evidence about the prime minister's alleged incompetence this could be "very damaging".

In his blog, Mr Cummings claimed the prime minister once planned to have donors "secretly pay" for renovation of his official Downing street flat.

Like several of his recent predecessors, Mr Johnson and his fiancee Carrie Symonds are living in the flat above No 11 Downing Street, which is larger than the one above No 10.

In March, Downing Street dismissed as "speculation" suggestions that refurbishments carried out last year would be paid for by Tory donors through a charity set up for this purpose, amid reports that the costs had spiralled out of control.

Mr Cummings said Mr Johnson "stopped speaking to me about the matter in 2020", after he told the prime minister the renovation plans were "unethical, foolish, possibly illegal and almost certainly broke the rules on proper disclosure of political donations if conducted in the way he intended".

He added he would be "happy to tell the cabinet secretary [Simon Case] or Electoral Commission" what he knew about the matter but that his knowledge is "limited".

The Electoral Commission says it is working to establish whether any of the spending on the flat needs to be examined within its own remit on political donations, and therefore needs to be reported and subsequently published.

Analysis: 'Cummings is skilful at damaging opponents'

By Damian Grammaticas

When he was sacked by Boris Johnson last year Dominic Cummings left Downing Street ostentatiously carrying a cardboard box designed for office files.

Mr Cummings is a controversial figure. Many will not like him or put store by what he says. But the knowledge he bore with him is starting to come out as allegations.

And he says he's prepared to give evidence under oath.

When he gets his teeth into a cause, Mr Cummings is a tenacious campaigner, and he's skilful at weaving a narrative that can be damaging to his opponents. The prime minister is now one of them.

What may have the biggest impact is the way Mr Cummings is drawing together the disparate strands of recent lobbying leaks and revelations, into one arc, questioning Mr Johnson's "competence and integrity".

Downing Street says the prime minister has done nothing wrong. But can it close Pandora's box?

In response to Mr Cummings's claims, a No 10 spokesperson said: "At all times, the government and ministers have acted in accordance with the appropriate codes of conduct and electoral law.

"Cabinet Office officials have been engaged and informed throughout and official advice has been followed.

"All reportable donations are transparently declared and published - either by the Electoral Commission or the House of Commons registrar, in line with the requirements set out in electoral law.

"Gifts and benefits received in a ministerial capacity are, and will continue to be, declared in transparency returns."

Responding to a Parliamentary question, Cabinet Office minister Lord True said that during successive governments the prime minister has been given an allowance of £30,000 a year from the public funds to maintain and furnish the flat in Downing Street.

He said no work took place in 2019-20 due to the pandemic, but official figures on spending this year are not yet available.

Downing Street contractors have carried out work on painting, sanding and floorboards, Lord True said, while "any costs of wider refurbishment in this year have been met by the prime minister personally".