Boris Johnson’s claim he knew nothing about who was funding the refurbishment of his Downing Street flat has been contradicted by an investigation.

The Electoral Commission has fined the party £17,800 for improperly declaring donations by Huntswood Associates used for the works at 11 Downing Street, where the PM lives.

In its report, concluding an investigation into the payments, the commission revealed the PM sent Lord Brownlow, the multimillionaire director of Huntswood, a WhatsApp message requesting funding for the refurb in November 2020.

But in May this year, Lord Geidt, the independent adviser on ministers’ interests, was told by the PM he did not know who was behind the flat refurb until February 2021, just before reports emerged in the media.

As part of the investigation, the commission looked at bank statements and WhatsApp messages from people involved.

The Electoral Commission report says Lord Brownlow paid the firms who carried out the refurb directly, although the Conservative Party told Sky News in April that was “nonsense”.

Mr Johnson’s wife, Carrie Johnson, is believed to have chosen £850-a-roll gold wallpaper by top designer Lulu Lytle, who co-founded exclusive decorating company Soane Britain.

The investigation found decisions relating to the handling and recording of a donation by Huntswood “reflected serious failings in the party’s compliance systems”.

The party’s fine is nearly the maximum allowed amount, which is £20,000.

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Lord Brownlow, who rubs shoulders with royalty, has donated close to £3m over the past six years – both personally and through his business, records show.

The commission found the party failed to report the full value of a donation of £67,801.72 from Huntswood in October 2020, with £52,801.72 connected to the costs of the refurbishment.

Prime ministers are given a £30,000 a year allowance to refurbish and maintain the Downing Street flat, with any extra costs to be met by themselves.

The commission also found the reference in the party’s financial records to the payment of £52,801.72 that was made by the part for the refurbishment “was not accurate”.

It said the fine for failing to accurately report the full value of the donation is £16,250 and for contravening the requirement to keep proper accounting records the party has been fined £1,550.