Boris Johnson is under pressure to hold an inquiry into the refurbishment of his Downing Street flat following claims by his former chief adviser Dominic Cummings.
In a scathing blog post, Mr Cummings accused his former boss of plotting an “unethical, foolish, possibly illegal” plan to get Tory donors to secretly fund the work.
Labour leader Sir Keir Starmer said “it stinks”, while a former Tory government law officer is calling for transparency.
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.
They want to establish if there were any conflicts of interest amid the continuing lobbying scandal at Westminster.
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.
Labour leader Sir Keir said: “It matters. It is about integrity, it is about taxpayers’ money. Every day there is more evidence of this sleaze. Frankly, it stinks.
“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.”
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 added: “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.”
Scottish First Minister Nicola Sturgeon said: “The stench of sleaze that is surrounding this UK Tory Government is becoming quite overpowering.”