Liz Truss has been claiming from the £115,000-a-year public fund awarded to former prime ministers to run their offices, despite only serving for 49 days.
Cabinet Office accounts released on Tuesday show that the Conservative MP claimed £23,310 in her first five months out of office.
It was understood she has continued to claim in the current financial year that started in April, but the sum will not be disclosed until next year’s report.
Ms Truss’s office declined to comment.
The Liberal Democrats’ Cabinet Office spokeswoman Christine Jardine urged Prime Minister Rishi Sunak to “do the right thing and stop Liz Truss from claiming taxpayers’ cash from the ex-PM fund”.
“It’s an outrage that while families struggle to pay their bills and put food on the table, Liz Truss profits from her own failure,” she said.
“If Liz Truss wants to cut tax she should lead by example and stop taking hardworking British taxpayers for a ride by claiming handouts.”
The Public Duty Cost Allowance affords former prime ministers up to £115,000 a year to cover office and secretarial costs arising from public duties.
Applicable costs including those for running an office, handling correspondence as an ex-PM and for support with visits.
Sir Tony Blair and Sir John Major were the only former leaders to claim the maximum amount in 2022/23, though Gordon Brown was close on £114,627.
Ms Truss’s chaotic tenure in No 10 ended on October 25 after losing the support of Tory MPs.
She hit out at economists and “institutional bureaucracy” for her downfall as she hinted at further plans to intervene in Tory politics at the party conference next month.
The Cabinet Office accounts also detailed the total cost of Boris Johnson’s taxpayer-funded legal defence to the inquiry that found he lied to MPs over partygate.
The final cost was put at £263,079, in line with what the department had previously revealed.
The public spending watchdog questioned the decision to use taxpayers’ money for Mr Johnson’s lawyers.
National Audit Office chief Gareth Davies said he looked at the spending because while the amount was “not quantitatively material” in the wider scale of the Cabinet Office’s finances, there was “significant public interest as to whether these costs are a legitimate use of public money”.
He said the arguments put forward to justify the spending – including that similar legal support had been provided to ministers appearing before public inquiries – were not “wholly persuasive” and it was a “borderline” judgment.