Boris Johnson’s ethics adviser, Lord Geidt, has demanded an explanation after the prime minister was accused of misleading him over the Downing Street flat scandal, with sources suggesting Geidt could resign if the answer does not satisfy him.

Pressure is mounting on Geidt, the adviser on ministerial interests, over the investigation he completed in May. It exonerated Johnson of wrongdoing over an undeclared £52,000 donation to the Conservatives to cover the cost of a makeover to the prime minister’s No 11 residence.

Despite Geidt’s report finding that Johnson said “he knew nothing about such payments” until stories in the press emerged in February 2021, the Electoral Commission released new evidence in a report this week.

A WhatsApp message was unearthed sent from Johnson to the donor – Lord Brownlow – in November 2020 asking him to approve more funds for the costly refurbishment.

Sources predicted Geidt would not “act hastily” and quit immediately, but would probably consider his position if unconvinced by Downing Street’s explanation that Johnson knew Brownlow was administering the donations but not the source of them himself.

No 10 confirmed that officials were liaising with Geidt, who could potentially update his findings. A senior Whitehall source said Geidt was furious on Thursday after the publication of the Electoral Commission report but was considering No 10’s explanation of what happened. No 10 is likely to have argued that Johnson knew Brownlow was overseeing the organisation of funds for the refurbishment but was not aware he was the underlying donor.

Johnson’s official spokesperson refused to say whether Geidt and the prime minister had spoken personally.

The government insiders suggested Geidt’s resignation was a “realistic option” given how, they said, he valued his integrity – both personally and for the office he holds.

Geidt needs Johnson’s permission to formally reopen the case, so is expected to ask him to explain the apparent contradiction – either by seeking a statement from Downing Street, requesting a personal interview with the prime minister, or asking for new evidence such as the WhatsApp message he seemed unaware of before.

He may wish to update the initial report by adding an addendum or footnote, but any such changes would need the signoff of Johnson himself.

Geidt’s resignation would be an embarrassment for Johnson, who has spent months fighting off sleaze allegations.

Geidt, having only taken up the post in April, replaced another adviser on ministerial interests who also quit over Johnson’s actions. Alex Allan stood down in November 2020 after the prime minister overrode his findings that the home secretary, Priti Patel, had bullied staff.

Even if Geidt is not allowed to reopen his investigation, the parliamentary commissioner for standards, Kathryn Stone, is expected to launch an inquiry into the issue.

She assured Labour’s deputy leader, Angela Rayner, she would look into the flat-funding scandal and undeclared donation once the Electoral Commission had published its report.

Rayner has said Johnson “must now explain why he lied to the British public” and accused him of taking people for fools.

Although more than £110,000 was paid on Johnson’s behalf for the refurbishment works, the prime minister later covered the cost himself, meaning Brownlow’s money ended back in his own hands. The source of the funding used by the prime minister to cover the costs has not been revealed.