Lord Geidt has been criticised for assuming a Tory donor who funded Boris Johnson’s flat makeover had “altruistic motives”, as Labour claimed backbench MPs were being held to a higher standard than the Prime Minister.

       Angela Rayner, Labour’s deputy leader, ratcheted up pressure on the Prime Minister’s ethics adviser as she demanded he clarify whether he still believed Mr Johnson had not broken the ministerial code.

       In a letter sent to Lord Geidt, Ms Rayner challenged Lord Geidt’s finding that the donor funding of the Downing Street flat refurbishment did not represent a conflict of interest. Lord Geidt said there was no evidence he had “acted with anything other altruistic and philanthropic motives”.

       She also claimed that his judgment appeared to “set a standard” which did not comply with the wording of the ministerial code. She also said it appeared to be “weaker” than the tests that apply to MPs.

       “This suggests that you will hold ministers to a lower standard of transparency than backbench MPs,” said Ms Rayner.

       It comes after Mr Johnson faced allegations of a “wallpaper for access” scandal when it emerged on Thursday that he had agreed to consider proposals for a new festival while discussing the refurbishment of his Downing Street flat with Lord Brownlow of Shurlock Row.

       In WhatsApp messages disclosed by Lord Geidt, Lord Brownlow - who originally provided funds as part of the ￡112,000 makeover of the flat - discussed his plans to hold a new Great Exhibition festival with the Prime Minister.

       Mr Johnson agreed to look at the proposal.

       Three months later, Lord Brownlow met Oliver Dowden, then Culture Secretary, and representatives from the Royal Albert Hall to discuss the plan, although it ultimately failed to materialise.

       The peer is a trustee of the Royal Albert Hall, which was built in the wake of the first Great Exhibition in 1851, during the reign of Queen Victoria.

       The messages were not disclosed to Lord Geidt when he originally investigated the flat makeover last year, but were later unearthed as part of an inquiry by the Electoral Commission in November.

       Despite revisiting the issue, Lord Geidt said in a series of letters published on Thursday that he stood by his original conclusion that there had been no conflict of interest.

       Challenging his findings on Thursday, Ms Rayner said: “WhatsApp messages clearly show the discussion between the Prime Minister and Lord Brownlow about the release of funds also covered the 'Great Exhibition 2.0' project in which Lord Brownlow had a personal interest.

       “Do you stand by your original conclusion that there could be 'no reasonably perceived conflict' in the Prime Minister seeking the support of a donor who at the same time is lobbying him for government support of a project they are promoting?

       “It is irrelevant in this regard whether Lord Brownlow’s motives are altruistic; the issue is that a reasonable person could surely perceive that his financial relationship with the Prime Minister has provided him with privileged access to government, and that relationship was undeclared at the time.

       “Indeed, it is obvious from the exchanges that the personal support of the Prime Minister, in his official capacity, is exactly what Lord Brownlow sought.”

       While Downing Street believes Lord Geidt’s letters have drawn a line under the controversy, Ms Rayner also urged him to consider issuing a new or amended report.