Cabinet secretary Simon Case has been asked to investigate whether Liz Truss inappropriately used a grace-and-favour mansion for a Conservative leadership election campaign meeting.
Truss’s use of the Kent country house Chevening, which is made available to the foreign secretary of the day, could be a breach of the ministerial code, which says public resources should not be used for political activity.
Lib Dem chief whip Wendy Chamberlain has written to Case, asking him to examine what happened and whether Truss broke the code.
In the letter to Case, she said: "It would be wholly inappropriate for the foreign secretary to be using her official residence for campaigning purposes at the taxpayer's expense.”
The ministerial code says ministers who hold personal events at residences should do so at their own or their party's expense, with no cost falling on the public purse. It also says government property "should not generally be used for constituency work or party political activities".
The Cabinet Office confirmed it has received the letter and will reply to the Lib Dems.
The claim from the Lib Dem MP came on the same evening as Truss suggested at a hustings that she may not appoint a new independent ethics adviser because she understands the “difference between right and wrong”.
Christopher Geidt, who quit the role in June after saying he was placed in an “impossible and odious position”, would normally have been charged with investigating potential breaches of the ministerial code. Case and Cabinet Office propriety and ethics chief Darren Tierney have temporarily taken over this role.
Instead of giving a clear answer when asked whether she would appoint an independent adviser on ministerial interests, Truss said on Tuesday evening that she would “ensure the correct apparatus is in place so that people are able to whistle-blow”.
Speaking at the leadership debate in Birmingham, she said: “I do think one of the problems we have got in this country in the way we approach things is we have numerous advisers and independent bodies, and rules and regulations.
“For me it’s about understanding the difference between right and wrong, and I am somebody who has always acted with integrity… and that is what I would do as prime minister.”
The allegations from the Lib Dems that she may have broken the ministerial code came after the Sunday Times reported last week that Truss was planning to take a team of senior aides to Chevening House to get her policies and cabinet team more firmly decided if she wins the election.
Chamberlain told the Independent: "It is incredibly important that we find out right away if the foreign secretary is using the public purse for her own leadership ambitions."
Truss' campaign has been approached for comment.
This is not the first controversy surrounding the campaign that Case has been asked to dig into.
In July he launched an investigation into leaks to the media after an MP raised concerns that information may have been shared to influence the Tory leadership race.
The investigation was thought to concern a leak to the Sunday Times of documents about changes to gender self-identification policy, which were drawn up by civil servants when erstwhile Tory leadership contender Penny Mordaunt was equalities minister, the Telegraph reported.