A TORY minister has defended Boris Johnson’s decision to remain the final arbitrator of any standards investigations, including over the renovation of his Downing Street flat.

Nadhim Zahawi insisted it is “only right” that the newly-appointed ministerial standards adviser reports to the Prime Minister as Labour criticised the arrangement for meaning Johnson “effectively marks his own homework”.

Lord Geidt’s review into whether any donations for the refurbishments were properly declared became one of three under way after the Electoral Commission launched a formal investigation, saying there are “reasonable grounds” to suspect an offence.

Downing Street has said Johnson would be “happy” to assist the watchdog’s inquiries into who initially paid for the work and whether any donation was properly declared, as the Prime Minister insisted he has not broken any laws or rules.

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But Labour has warned Lord Geidt’s position is not truly independent because Johnson continues to be the “ultimate arbiter” of the ministerial code, therefore allowing the Prime Minister to be “judge and jury” of his own behaviour.

But Vaccines Minister Mr Zahawi, backing the crossbench peer as a “really creditable individual”, issued a defence of the system.

“All ministers serve with the confidence of the Prime Minister – I think that’s right, that is the right system to have,” he told BBC Breakfast.

“So it is only right that the adviser on ministerial conduct has to be able to report to the Prime Minister.”

Lord Geidt was appointed to the position yesterday, five months after the resignation of his predecessor Sir 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”.

Lord Geidt does not have the power to launch his own investigations and No 10 confirmed that the Prime Minister remains the final adjudicator of any breaches.

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Labour’s shadow chancellor of the Duchy of Lancaster Rachel Reeves said: “The Prime Minister can’t be judge and jury on his ministers’ – or indeed his own – behaviour.

“The Prime Minister shouldn’t be able to block investigations into his ministers or himself when breaking the ministerial code.”

Lord Geidt has been tasked with “ascertaining the facts surrounding” the renovation and advising Johnson “on any further registration of interests that may be needed”.

In separate inquiries, Electoral Commission investigators can demand documents and information, and could potentially seek a statutory interview with the Prime Minister as part of the process.

Cabinet Secretary Simon Case, who is head of the Civil Service, is also undertaking a review.

Johnson has said he “personally” paid for the renovations but has refused to say whether he received an initial donation from the Conservative Party to cover the costs reported to be up to £200,000.

Chancellor Rishi Sunak has made it public that he paid “upfront and entirely at his own expense” when he redecorated his own Downing Street flat last year.

Questions have been mounting over the flat since former aide Dominic Cummings accused Johnson of wanting donors to “secretly pay” for the renovations to the apartment in a “possibly illegal” move.

The Electoral Commission said its investigation will “determine whether any transactions relating” to the renovations “fall within the regime regulated by the commission and whether such funding was reported as required”.

The Conservative Party said it would “continue to work constructively” with the commission.

“We believe all reportable donations have been transparently and correctly declared and published by the Electoral Commission,” a spokesman said.

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Prime ministers get a budget of up to £30,000 per year to renovate their Downing Street residency, but newspaper reports have suggested Johnson’s costs have spiralled.

Tory peer Lord Brownlow said in an email leaked to the Daily Mail that he was making a £58,000 donation to the Conservatives “to cover the payments the party has already made on behalf of the soon-to-be-formed ‘Downing Street Trust’”.

The appointment of former private secretary to the Queen Lord Geidt as the new adviser paves the way for the publication of the latest register of ministerial interests, which could contain details of any donations to fund the flat.