Boris Johnson is under pressure to explain how the lavish refurbishment of his Downing Street flat was paid for following an explosive attack by his former chief adviser Dominic Cummings.

In an incendiary 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.

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.

However, Labour said that Mr Johnson needed to explain how he obtained the money in the first place to pay for the work overseen by his fiancee Carrie Symonds.

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.

Shadow communities secretary Steve Reed said the Government must now publish any correspondence relating to payments or donations around the refurbishment.

“We need to know the full amount that was spent and we need to know who paid for the work in the first place, who the Prime Minister now proposes to reimburse,” the Labour MP told the BBC Radio 4 Today programme.

“If people are making significant donations to the Conservative Party, to the Government, we need to know who they are so that we can make sure the Government isn’t doing favours for them in return.

“That is what sleaze is all about. That is the behaviour of a tin-pot dictatorship. Britain, frankly, deserves a lot better than that.”

Conservative former attorney general Dominic Grieve – a long-standing critic of Mr Johnson – also called for transparency, describing the Prime Minister as a “vacuum of integrity”.

“It is all smoke and mirrors. He hasn’t said when he decided to repay it or whether he has now repaid it,” Mr Grieve told the 4 Today programme.

“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 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 over tax matters.

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 it could implicate Henry Newman, a close friend of his fiancee.

“It is sad to see the PM and his office fall so far below the standards of competence and integrity the country deserves,” he said.

On Friday, the Prime Minister denied trying to block the leak inquiry, saying the public could not “give a monkey’s” about such matters.

In relation to the refurbishment of the flat, Downing Street said that 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.

A commission spokeswoman said: “Discussions with the Conservative Party continue as we work to establish whether any sums relating to the works at 11 Downing Street fall within the regime regulated by the commission, and therefore need to be reported and subsequently published.

“The party is working with us on this.”