Liz Truss has been claiming cash from the £115,000-a-year public fund awarded to former prime ministers to run their offices, despite serving for just 49 days.
Cabinet Office accounts released on Tuesday show the Conservative MP claimed £23,310 in her first five months after resigning.
It is 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.
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The Lib Dems said it was an “outrage” that while families are struggling in a cost of living crisis, “Liz Truss profits from her own failure”.
Ms Truss became the the shortest-serving prime minister in British history when she resigned in October last year after her disastrous mini-budget sparked economic turmoil.
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Her £45bn package of unfunded tax cuts, which she admitted would primarily have benefited the wealthy, sent the pound tumbling, interest rates soaring and culminated with the Bank of England having to intervene to prevent pension markets from collapsing.
After she stepped down, Labour leader Sir Keir Starmer was among those arguing that she should decline the allowance given to former PMs, saying she had “not earned the right to that entitlement”.
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 include 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.
The Liberal Democrats’ Cabinet Office spokeswoman Christine Jardine said Rishi Sunak should intervene to stop Ms Truss “from claiming taxpayers’ cash” from the fund.
She said: “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.
“If Liz Truss wants to cut tax she should lead by example and stop taking hardworking British taxpayers for a ride by claiming handouts.”
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It comes after Ms Truss defended her mini-budget in an unrepentant speech at an Institute for Government event on Monday.
While she admitted trying to do too much too fast, she did not apologise as she hit out at economists and “institutional bureaucracy” for her downfall.
She also hinted at further plans to intervene in Tory politics at the party conference next month.
