Boris Johnson will retain the power to exonerate himself of any possible breach of the ministerial code, including over the refurbishment of his Downing Street flat.

No 10 said the Prime Minister would remain “ultimate arbiter” of the code after announcing the long-awaited appointment of a new independent adviser on ministers’ interests.

The appointment of Lord Geidt, a crossbench peer who was the Queen’s private secretary for a decade, came as the Electoral Commission launched a formal investigation into the renovation.

Lord Geidt was tasked with “ascertaining the facts surrounding” the renovation of the No 11 residence and advising Mr Johnson “on any further registration of interests that may be needed”.

The Prime Minister’s official spokesman said “one of the first” tasks for the new adviser is to look at publishing the long-delayed list of ministers’ interests, which has not been updated since July and could contain details of any donations to fund the Downing Street flat.

The post had been vacant since Sir Alex Allan resigned in November in response to Mr Johnson standing by Priti Patel, despite an investigation finding the Home Secretary’s conduct “amounted to behaviour that can be described as bullying”.

No 10 changed the role to allow the adviser to raise with the Prime Minister potential breaches of the ministerial code suitable for investigations.

But the Prime Minister’s official spokesman said Mr Johnson has concerns that an adviser with powers to independently launch probes could be drawn into an investigation with “trivial or vexatious complaints”.

Asked if that means he could reject any findings on himself, the spokesman said: “The Prime Minister will remain the ultimate arbiter of this, yep.”

Lord Geidt’s work will be separate to a review into the matter by Cabinet Secretary Simon Case, which brings the number of investigations along with the Electoral Commission’s to three.

The appointment was also changed to make it a non-renewable five-year term and to allow the holder to advise on appropriate sanctions, Downing Street said.

Lord Geidt, in a letter to Mr Johnson, said he welcomed the moves to “stiffen the degree of independence of the post” and said he looks forward to “playing a part in maintaining the integrity of the ministerial code”.

His present responsibilities include being chairman of King’s College London and chairman of international relations and corporate responsibility at Schroders plc.