Liz Truss has been reported to the Cabinet secretary amid claims she is using her grace-and-favour mansion to help with her leadership bid.
On Sunday claims emerged that the Tory leadership frontrunner was using Chevening for a campaign team meeting – which would be against the rules.
Opposition MPs have called for an investigation to be launched into the affair and warned Ms Truss could end up looking like "continuity Boris" on sleaze.
If confirmed, using the Grade I-listed 17th Kent country house could constitute a breach of the ministerial code, which forbids political activity with public resources.
It comes after questions were raised earlier this month about why a separate “Fizz with Liz” Champagne dinner was not declared on Ms Truss’s register of interests.
A letter to Cabinet secretary Simon Case by Lib Dem chief whip Wendy Chamberlain warns that the use of the house by the Liz for Leader campaign "could constitute the use of Chevening for party political activities".
"It would be wholly inappropriate for the Foreign Secretary to be using her official residence for campaigning purposes at the taxpayer's expense," it adds.
"I therefore urge you to launch an investigation into whether the Foreign Secretary has broken the Ministerial Code."
Mr Case is overseeing the ministerial code following the resignation of Lord Geidt, who quit as chief ethics advice over Mr Johnson's approach to government.
The Ministerial Code states that “Government property should not generally be used for constituency work or party political activities".
On the matter of official residences the code states that "where Ministers host Party or personal events in these residences it should be at their own or Party expense with no cost falling to the public purse".
The Sunday Times newspaper reported over the weekend that Ms Truss was taking aides to the manor house to nail down her final policies and decide who her top team would be if she wins the contest.
Ms Truss and her campaign team have yet to comment on the affair.
"Liz Truss and Rishi Sunak are continuing their leadership melodrama, whilst around the country people are suffering due to the cost of living crisis, and it is only going to get worse," MP Ms Chamberlain said .
"Therefore it is incredibly important that we find out if members of the current Zombie Government are using the public purse for their own leadership ambitions," she said.
“Boris Johnson trashed standards in public life. We know that Truss wants to be seen as continuity Boris, but lets hope she has not followed his lead in trashing the Ministerial Code and taking the British people for fools”
It is incredibly important that we find out right away if the Foreign Secretary is using the public purse for her own leadership ambitions.”
The Cabinet Office said it had not yet received the letter and declined to immediately comment on the matter.
Chevening is owned by a public trust which is exempt from some taxation. It was gifted to the government by its previous hereditary owner in the 1950s for official use.
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