A Government Whip was ordered to apologise and repay more than £4,000 in parliamentary expenses today.
Tory MP Bill Wiggin put in inflated claims for maintenance on his second home and phone lines that were also being used by his wife's business.
The Commons Standards and Privileges Committee also criticised "chaotic" record keeping that made it impossible to establish for certain what had been spent as part of his parliamentary duties.
The report by the powerful watchdog came after a probe lasting nearly a year into Mr Wiggin - a contemporary of David Cameron's at Eton.
Between 2004 and 2006 the Leominster MP regularly filed £240 claims for council tax, telephone, utilities, service and maintenance - £10 below the threshold where receipts were needed under the rules at the time.
Mr Lyon concluded that the level of claims was probably influenced by the fact he did not have to supply receipts.
Referring to Mr Wiggin's decision to install three telephone lines into his second home and share them with his wife's business, Mr Lyon commented: "It would have made much more sense for there to have been separate lines separately billed and charged."
Mr Lyon said the MP had supplied "limited and largely circumstantial evidence" to support his expenses, despite repeated requests for more documentation.
But the commissioner was still able to conclude that he had claimed £285 too much for council tax, and it was "more likely than not" his claims for telephone, utilities, service and maintenance were unjustified.
The cross-party committee agreed with the commissioner that the breaches were "serious" - although it stopped short of deciding Mr Wiggin's utilities claims were too high after he provided extra material.
"The evidence shows that it is more likely than not that Mr Wiggin did claim for expenditure he did not incur, in respect both of telephone services and of service and maintenance," the committee said.
"However, the new evidence supplied by Mr Wiggin does not support a conclusion that he over-claimed for gas and electricity costs.
"Although these costs appear to have been remarkably high, we conclude that Mr Wiggin was within the rules in claiming, as he did, £240 each month towards them from his Additional Costs Allowance."
The watchdog ordered Mr Wiggin to apologise in writing for "muddled" and "chaotic" arrangements that saw him designate his constituency property as his taxpayer-funded residence between 2004 and 2007 - but then claim expenses against his London home.
However, they accepted the mistake was "unfortunate and unintended". "Mr Wiggin gained nothing and the public purse lost nothing from this mistake," the report added.
"We further recommend that Mr Wiggin repay the £285 he over-claimed in respect of council tax in 2005/06.
"We recommend that Mr Wiggin apologise to the House through this Committee in writing for claiming from Parliamentary allowances costs that we have concluded he did not actually incur and that he repay £4,009, representing half of the sum he was paid from Parliamentary allowances in respect of telephone and communication costs and service and maintenance costs for his second home in financial years 2004/05 and 2005/06."