Boris Johnson has said there is nothing to “worry about”, after a formal investigation was launched into how the refurbishment of his Downing Street flat was paid for.

Speaking to broadcasters on Thursday morning, the prime minister said he would “comply” with the Electoral Commission inquiry.

“I don’t think there is anything to see here, or worry about,” he said.

The probe by the elections wathdog will seek to establish who initially paid for the work and whether any donation was properly declared.

Johnson has said he has now personally “covered” the cost of the work. But he has repteadly avoided clarifying who initially paid for it.

Downing Street has refused to say whether Johnson first received an loan from the Conservative Party.

The row was triggered after Dominic Cummings, Johnson’s former top adviser, accused the PM of wanting donors to “secretly pay” in a “possibly illegal” move.

Johnson also said on Thursday that newly-appointed ministerial standards adviser, Lord Geidt, will do an “outstanding job” in his own review into whether any donations were properly declared.

But Labour has criticised the arrangement because Johnson remains the “ultimate arbiter” of the code.

The Opposition said it meant the prime minister “effectively marks his own homework”.

Keir Starmer said the situation was getting “a bit farcical”.

He demanded Johnson “answer a very simple question” on who initially paid for the refurbishments. “What is he hiding?,” the Labour leader added.

Johnson argued, in a letter to chairman of the committee on standards in public Life Lord Evans, that he “cannot and would not wish” to give up the power over the code.

“That vital responsibility is quite properly mine alone and, as an elected politician, one for which I am ultimately accountable to the electorate,” he said.

Lord Geidt was appointed to the position on Wednesday, five months after the resignation of his predecessor Alex Allan.

Allan quit in response to Johnson standing by Priti Patel despite an investigation finding the home secretary’s conduct “amounted to behaviour that can be described as bullying”.