British Prime Minister Boris Johnson faces questions about the costly refurbishment of his apartment on London’s Downing Street following a series of incendiary allegations made by his former top adviser.

In a wide-ranging blog post late on Friday, Dominic Cummings accused his former boss of attempting an “unethical, foolish, possibly illegal” plan to get Conservative Party donors to fund a lavish refurbishment of the apartment, in which he lives with fiancee Carrie Symonds and their baby son, Wilfred.

The charge is the latest in a string of allegations that Mr Johnson and other Conservatives, including former prime minister David Cameron, blurred or broke rules relating to contacts with donors and business.

Follow the money

The government said on Friday that Mr Johnson himself met the cost of the flat refurbishment, thought to be about £200,000 ($A358,000).

But the opposition Labour Party said the Prime Minister needed to explain how he obtained the money to pay for the work overseen by Ms Symonds.

“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,” Steve Reed, Labour’s communities spokesman, told BBC radio.

Mr Cummings made a series of other allegations against Mr Johnson in the blog post, including that the Prime Minister sought to halt an investigation into the leak of plans for a second coronavirus lockdown after he was warned a close friend of Ms Symonds could be implicated.

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

Asked on Friday by Britain’s LBC radio if he had tried to block the leak inquiry, Mr Johnson said: “No, of course not.”

Allies become enemies

In a statement, Downing Street said the Prime Minister had “never interfered” in the inquiry and the government had “acted in accordance with the appropriate codes of conduct and electoral law” in funding the flat refurbishment.

Mr Cummings, who along with Mr Johnson played a key role in the successful 2016 Brexit campaign for Britain to leave the European Union, resigned his job at Downing Street in November.

The relationship between Mr Cummings and Mr Johnson has clearly soured.

Mr Cummings’ blog post followed reports Mr Johnson believes his former adviser was behind a series of leaks, including of text message exchanges between the PM and businessman James Dyson.

The British government faces growing sleaze allegations largely linked to Mr Cameron, who was prime minister from 2010-2016. A series of inquiries has been launched into Mr Cameron’s lobbying role for Greensill Capital, a financial firm that collapsed in March.

-AAP