The Telegraph understands Lord Geidt has asked the Prime Minister and Lord Brownlow to provide an explanation as soon as Friday of why the WhatsApp messages were not divulged to him along with other evidence when he looked into the matter.

If Lord Geidt, a former private secretary to the Queen, does not receive what he regards to be a satisfactory explanation, The Telegraph understands that he is likely to resign because he would not be able to do his job if the bond of trust between him and Mr Johnson were to be broken.

Separately, there is mounting speculation that Kathryn Stone, the Parliamentary Commissioner for Standards, will now initiate her own investigation into Mr Johnson.

While Ms Stone’s office has refused to comment, Labour has written to the commissioner demanding that she investigate, with parliamentary sources suggesting a decision on whether to do so could be announced early next week.

Publishing the findings of its investigation, the Electoral Commission said the Conservatives had failed to fully report a donation of £67,000 from Huntswood Associates Limited, a company controlled by Lord Brownlow.

While the Conservatives only reported £15,000, the commission said the remaining £52,000 that was used to fund the renovation of the flat in Number 11 Downing Street should have been disclosed. This amounted to breaches in the law around the reporting of donations, it concluded, and represented “serious failings” in the party’s “compliance systems”.

It comes eight months after the investigation was first launched on the back of a string of reports into the lavish flat renovation and mounting scrutiny over the failure of the Conservative Party and Mr Johnson to register the funds in public transparency registers.

The report found that, in total, Lord Brownlow paid £112,549 towards the renovation of the flat. It also disclosed that, in October last year, a junior member of Conservative staff asked about the £52,801 the party had not declared, and were told: “Don’t worry.”