Disgraced MP Eric Joyce was putting hotel bills of up to £450 a night on a taxpayer-funded credit card for more than two years before it was blocked.
The ex-Labour backbencher - who is standing down at the election after admitting assaulting a Tory counterpart in a Commons bar - repeatedly used his card to settle big bills that were later declared unclaimable on expenses.
But the Independent Parliamentary Standards Authority (Ipsa) did not withdraw it until this January - by which time the spending had helped him run up a total debt of some £13,000.
Mr Joyce is now being docked £1,000 a month from his Commons salary but that will not clear the outstanding sum of around £10,000 before Parliament is dissolved.
The Falkirk MP accepts that he made mistakes in filing claims after using his card, suggesting he was also paying for rooms for staff.
However, he is disputing at least £6,000 of the debt. If he refuses to repay voluntarily the watchdog's only option will be to pursue him in the civil courts.
The details emerged after Ipsa admitted it has been labelling some of Mr Joyce's hotel claims as "not paid" in its regular disclosures - when in fact it has paid a much higher bill in full.
The watchdog confirmed he put £743 on a card to fund a three night-stay at a "non-London" hotel, believed to be in Glasgow, in September last year.
The bill was automatically paid by Ipsa. But as he is only entitled to claim up to £120 a night for hotels outside the capital, an excess of £383 was added to his debt.
The published expenses records merely state that £383 was "not paid". He was was "not paid" £272.33 for a non-London hotel claim in August, and £208 for another in September.
Mr Joyce stayed at a prestigious Intercontinental Hotel in London in May 2013, when he appears to have broken the £150 a night limit for hotels in the capital by £203.05.
He used the same group again the following month, but this time breached the ceiling by just £61.45.
The issues seem to go back to at least 2012. In December of that year - 10 months after he headbutted a Tory MP during a brawl in a Commons bar - he charged £896.92 to the card for a two-night stay at a London hotel.
It is understood that the claim was declared invalid because key parts of the receipt had been blacked out - although again Ipsa had already footed the cost and subsequently reported it as "not paid".
Altogether there are dozens of hotel claims listed as either "not paid" or "part paid" since 2012, totalling some £4,000.
Mr Joyce insisted he was using hotels booked through the parliamentary travel office, and said some bills were high because staff stayed at the same time. The £896.92 bill was for a "whole bunch of people" to stay at the Park Plaza near Westminster, he said.
"That was all of my staff in London," he said. "That was essentially an error. It should have come off my (personal) card.
"They would have all been eligible (for expenses) but the parliamentary card was used rather than their cards or my card."
Mr Joyce said Ipsa had contacted him about spending in late 2013, but he heard nothing else and believed they had dropped the matter until last December.
"The whole figure was only exposed in December," he said. "Some of these figures are legitimate debts to Ipsa but they would not discuss the rest of it."
Mr Joyce said there was a "particular issue" with a member of staff who moved to Brighton from Scotland.
He said the watchdog had signed off an arrangement where the employee worked from home but occasionally stayed at hotels in London.
However, after 16 months, Ipsa decided the claims were invalid and demanded the money back, according to Mr Joyce.
The MP said he had asked the watchdog's compliance officer to look again at the situation.
An Ipsa spokesman said: "MPs have a responsibility to manage their financial affairs and we publish details of all of their claims - both paid and not.
"We have consistently told the MP that he has a budget to keep to and that he is liable for any excess. We have stressed this time and again throughout the Parliament, as we do to every MP.
"We've been talking to the MP since autumn 2013 to put his financial affairs on an even keel. As you would expect, when this didn't work we escalated these conversations within Ipsa and this approach resulted in a personal assurance from the MP that he would repay the amounts due.
"When those assurances were not followed through, we turned off his payment card in January 2015 and in February 2015 we moved to recovering money directly from his salary. We will continue to recover this money until it is repaid in full.
"This is a very unusual case. We've tried to encourage the MP to manage this situation. Clearly, this hasn't worked and we are committed to getting this money back. That is why have moved to more direct measures of recovery, including taking money straight from his pay packet."
In 2008, before Ipsa took over the system, Mr Joyce had his parliamentary expenses suspended after putting nearly £5,000 in "non-parliamentary" travel expenses to Tokyo, Kinshasa and Nairobi on a Commons card, and then ignoring repeated demands from officials to repay.
The latest situation could fuel calls for Ipsa to publish fuller, receipt-level information about MPs' spending.
The watchdog has already lost two rounds of an Freedom of Information battle to avoid publishing copies of every receipt filed, a process that could potentially cost millions of pounds.
The Court of Appeal is due to hear the case later this month - but no judgement is expected until after the election.