There are growing calls for Boris Johnson to explain how the refurbishment of his Downing Street flat was paid for.
The prime minister considered a "secret plan" to get Tory donors to foot the bill, according to his former adviser Dominic Cummings.
Labour is poised to seek an urgent question in Parliament this week to force a senior minister to face MPs in the Commons about Mr Cummings’ claims.
Asked about the move, opposition frontbencher Jess Phillips told Sky news’ Sophy Ridge On Sunday programme: "I believe so. I imagine that’s the case."
Number 10 insists no laws or ministerial codes were broken – while the PM said the public "don’t give a monkeys" about who pays for what.
Backing calls for an inquiry, Scottish First Minister Nicola Sturgeon condemned the "stench of sleaze" surrounding Mr Johnson’s government, which she said is "becoming quite overpowering".
Labour leader Sir Keir Starmer also declared that "it stinks", while a former Tory government law officer also waded into the row calling for transparency and branding Mr Johnson "a vacuum of integrity".
The government said for the first time on Friday that the prime minister paid for the revamp, reported to have cost £200,000, out of his own pocket.
But critics argue Mr Johnson needs to explain how he got the money in the first place to pay for the overhaul overseen by his fiancee Carrie Symonds, and whether there were any conflicts of interest, coming amid the continuing lobbying scandal at Westminster.
Transparency campaigner Gina Miller told Sky News: "We have politicians now who are not acting with integrity and honesty but we have a system where the checks and balances and the mechanisms within government are not very strong, so they’re not really breaking the law, because we don’t really have any laws.
"So it’s much more about putting in place laws so when we do have rogue politicians they can be held to account.
"I’m not quite sure why people are particularly surprised by what’s happening: this is the Boris Johnson who, as a journalist decades ago, had a very distant relationship with the truth, Bullingdon Boris who had a chaotic private life – none of this is particularly surprising: he is being Boris.
"It is lack of check and balances and legal responsibility in our system that is the conversation we should be having."
Meanwhile, the Electoral Commission has said it is still seeking answers from the Conservative Party over whether any sums relating to the work should have been declared under the law on political donations.
Former chairman of the Committee on Standards in Public Life Sir Alistair Graham told Sky News: "I’ve always taken the view on house refurbishments, that you have the money in the bank first before you start the refurbishment.
"So this seemed a very odd way of going about refurbishing a flat, having the refurbishment done at considerable cost and then rushing around trying to find suitable people to pay for it.
"I’m sure people will want to see the actual receipts of him having paid for this because there clearly was a strong story around earlier on that he was looking for donors to the Conservative Party to pay for this."
Speaking about Mr Cummings, he went on to say: "What will matter in the end [is] how much hard evidence has he got? He said he’s willing to give evidence under oath, that he’s willing to talk to the Electoral Commission, so one suspects that he knows where all the bodies are buried and he’s got something to back it up, otherwise why would he make these statements if they can be easily disproved?"
Conservative former attorney general Dominic Grieve, a long-standing critic of Mr Johnson, said: "The fact is that he did get, I think it has become quite clear, a significant gift towards the refurbishment of the flat.
"My impression is there has been constant wriggling about the source of the money for this refurbishment."
The explosive claim by Mr Cummings was part of a wide-ranging attack after No 10 sources briefed newspapers the former aide was the source of a number of damaging leaks – including text messages exchanged between the prime minister and the entrepreneur Sir James Dyson about tax.
It follows Mr Cummings’ dramatic departure last year from No 10 amid the fallout of a bitter internal power struggle with Ms Symonds.
In his blog post, Mr Cummings accused Mr Johnson of seeking to stop an inquiry into the leak of plans for a second coronavirus lockdown after he was warned Henry Newman, a close friend of his fiancee, had been identified as the so-called "chatty rat".
On Friday, the prime minister denied trying to block the leak inquiry, saying the public could not "give a monkey’s" about such matters.
On the refurbishment of the flat, Downing Street said the government and ministers had "acted in accordance with the appropriate codes of conduct and electoral law" throughout.
However, the Electoral Commission said that a month after it first revealed that it had contacted the Tory Party over a reported £58,000 donation in relation to the flat, those talks were still continuing.
According to a leaked email obtained by the Daily Mail, the Tory peer Lord Brownlow wrote to the party’s head of fundraising last October informing him that he was making a donation.
He said that it included "£58,000 to cover the payments the party has already made on behalf of the soon to be formed ‘Downing Street Trust’ – of which I have been made chairman, as you know," he wrote.
To date, no such trust has been formed.