Conservative party accounts reveal a Tory donor provided almost £53,000 towards the cost of Boris Johnson's Downing Street flat refurbishment.
Figures confirm Lord Brownlow reimbursed the party for a "bridging loan" it initially provided for some of the works.
The costs were then settled by the prime minister, but it sparked a row over ministerial conduct.
A report by the standards' adviser said the PM had acted "unwisely".
Details published clarify the extent of the Conservative peer's financial involvement in the renovations which are thought to have cost up to £200,000.
Conservative Party accounts show its central office provided a "bridging loan" of £52,802 to cover the works after being invoiced by the Cabinet Office in June last year.
The party was "reimbursed in full" by Lord Brownlow in October, before Mr Johnson "settled the costs" incurred by the Conservative peer in March.
A Conservative spokesman said: "As stated, the party provided a short-term bridging loan that was reimbursed to the party in full."
Prime ministers receive an annual public grant of £30,000 to spend on their Downing Street living quarters, which are publicly-owned.
But a Cabinet Office report in July said more than £28,000 was spent on painting and sanding floorboards alone.
And invoices for works, including services of interior designer Lulu Lytle, pushed the total costs significantly beyond the limit of public funding.
Mr Johnson's former chief adviser Dominic Cummings claimed the prime minister had planned to solve this shortfall by having donors "secretly pay" for the work.
In May, an investigation by the prime minister's adviser on standards, Lord Geidt, cleared Mr Johnson of misconduct although it found he should have been more "rigorous" in finding out how the costs were covered.
While the report found Lord Brownlow had paid an invoice for some of the costs, it did not say how much he had put up to fund the refurbishment.
Labour's deputy leader, Angela Rayner, said questions are hanging over so-called "flatgate".
She said the prime minister was embroiled in a "web of conflicts of interests, secret loans and cover ups".