Liz Truss is contesting a Government bill relating to her use of the grace-and-favour country house she had access to as foreign secretary.
The former prime minister has been asked to pay about £12,000 for costs incurred while she spent time at the Chevening estate, the Mail on Sunday reported.
The invoice covers the period last summer when she was preparing for her short-lived premiership and is understood to have been received by Ms Truss several weeks ago.
Chevening, a 115-room country retreat in Kent, was left to the nation by the 7th Earl Stanhope after his death in 1967 and has traditionally been used by the foreign secretary of the day.
But the former prime minister maintains the vast majority of the invoice relates to official government business, for which she should not be liable, rather than personal expenses.
The ministerial code states that ministers must not allow the costs of party or personal events in grace-and-favour estates to fall to the public purse.
A source had told the Mail that Ms Truss used the residence as a “mini-No 10”, holding meetings with her inner circle which often turned into parties in the evening.
A spokesman for Ms Truss said: “Liz always paid for the costs of her personal guests at Chevening.
“The latest invoice contains a mixture of costs for her personally and costs for official government business with civil servants including (Cabinet Secretary) Simon Case and senior officials from other departments who met at Chevening during the transition preparations.
“The latter constitutes the majority of the bill. It would be inappropriate for her to pay the costs for officials as it would have breached the Civil Service Code for civil servants to accept hospitality during the leadership campaign. She has therefore asked for this to be billed separately.”
A Government spokesperson said: “Costs and funding relating to Chevening House are a matter for the Chevening Trust.
“Where appropriate, we work closely with them to ensure costs incurred are allocated accordingly.”