Boris Johnson under pressure to explain how he funded Downing Street luxury refurbishment

Boris Johnson is under pressure to explain how the lavish refurbishment of his Downing Street flat was paid for as the furore around cronyism and conflicts of interest in government grows.

Opposition parties have written to the prime minister demanding a formal investigation into the £200,000 makeover "given the gravity of the new accusations" against the PM.

Mr Johnson's own former special adviser Dominic Cummings on Friday had accused him of an "unethical, foolish, possibly illegal" plan to get Tory donors to secretly fund the work.

As one of his own former MPs declared the prime minister a "vacuum of integrity" who had turned Downing Street into a "lie machine", Labour said Mr Cummings's claims would feed perceptions of conflicts of interest at the top of government.

The new allegations come after a torrid political week for the prime minister in which it was revealed he promised a to "fix" a tax issue for Brexiteer industrialist James Dyson after being contacted by text message.

Though Mr Johnson claims there was nothing "remotely dodgy" about the episode, it came after questions about how ministers Matt Hancock and Rishi Sunak has responded to lobbying by their former boss David Cameron on behalf of Greensill Capital.

Scottish first minister Nicola Sturgeon on Saturday called for an inquiry into allegations of "contracts for cronies, donations for decorating and text messages for tax breaks".

Claiming there was a "stench of sleaze" around the government, Ms Sturgeon said: "It is time for the Tories to put all their dealings into the public domain and let them be properly investigated."

SNP Westminster leader Ian Blackford added: "There are now so many different parts to this Tory scandal that a comprehensive investigation into the workings of this deeply dodgy Tory government is the only answer."

Labour's shadow cabinet office minister Rachel Reeves meanwhile wrote to Mr Johnson demanding a "full investigation".

“Given we know it only takes a text message from a friend to get the full attention at the top of your government, many people will wonder what personal goodwill could be generated by a secret donation to the redecoration of your living quarters," she said.

“Any external financial aid to a prime minister’s lifestyle must of course be fully declared at the time and as the ministerial code makes clear, real and perceived conflicts of interest must be avoided.

“I believe there needs to be a full investigation given the gravity of the new accusations from your former chief adviser and the serious implications of other irregularities of this concerning episode.”

The Electoral Commission, which regulates political donations, has said it is seeking answers from the Conservative Party over whether any sums relating to the work should have been declared.

Earlier on Saturday Dominic Grieve, who served as Conservative attorney general until 2014, accused the prime minister of leading a "cronyistic cabal" at the heart of government.

"There's been a constant wriggling about the source of the money for this refurbishment and that's just one illustration of the chaos that Mr Johnson seems to bring in his wake," Mr Grieve told BBC Radio 4's Today programme.

"And the reason for that is because he is a vacuum of integrity, and this has been apparent for a very long time: apparent to my colleagues who I regret to say elected him or caused him to be elected as leader of the Conservative Party.

"And if you have somebody who's a vacuum of integrity, you're going to get this sort of story coming up over and over again. And some people may say it doesn't matter, but I think in the context of trying to ensure good governance and people's faith in democratic institutions, it's really an extraordinary way to conduct oneself."

Mr Grieve added: "I have no particular respect for Mr Cummings: both Mr Cummings and Mr Johnson smeared me in September 2019 when they turned the No 10 press office into a sort of lie machine.

"I was a member of the Conservative Party for over 44 years, I think. And I look at the political party which seems to me to be spiralling into something significantly different in terms of its integrity from anything I can ever remember."

Steve Reed, the shadow communities secretary, accused the PM of acting in the manner of a "tinpot dictatorship". The controversy over the flat refurbishment is also raging amid allegations by Mr Cummings that the prime minister considered stopping a leak inquiry because he feared it would find his fiancee’s friend was behind it.

It also emerged on Saturday that Eddie Lister, one of Mr Johnson's senior aides, had apparent commercial interests in Libya at the same time as being a director at the Foreign Office.

The PoliticsHome website reports that Lord Lister was a director of a company that wanted to build a "Hong Kong" in the conflict-hit country while at the same time as having a "key role in the operational and strategic leadership" of the UK government department overseeing foreign policy.

Downing Street said the peer, one of Mr Johnsons' closest advisors, had declared any relevant interests.