The Wintertons both represent Cheshire constituencies
Sir Nicholas and Ann Winterton unwittingly broke MPs' expenses rules when claiming for a second home, the Parliamentary standards watchdog said.
The husband and wife Tory MPs claimed an allowance for the rent on a second home which is owned by a family trust.
Standards commissioner John Lyon said the couple should have known the rules on second home allowances had changed.
He said he accepted the breach was accidental but said it should not have gone on as long as it did.
The Wintertons, who both represent constituencies in Cheshire, transferred their second home - a flat in London - to a trust, to which they paid rent of £21,600 per year in 2002.
They said they had agreed the arrangement with the Commons Fees Office at the time it was set up and would not have gone ahead unless this had been the case.
In a statement issued when the probe was launched, the two MPs said they had been following advice from their solicitor and accountants about their likely inheritance tax liability.
They said they no longer owned the flat but were obliged by the trust to pay the full market rent recommended by an independent valuer.
But the standards committee report said the family trust, which had become the Wintertons' landlord when the property was transferred, "was an organisation in which they had an interest (as trustees) and their children had an interest (as the beneficiaries)".
And it adds: "Given their substantial personal interest in the arrangements, it would therefore have been prudent for the Wintertons, once having gone ahead with the arrangements, to have checked to ensure that charging the rent to the ACA remained consistent with the rules, particularly when these were changed."
The committee said that in its view the Wintertons' arrangements "have been in breach of the rules applying to the Additional Cost Allowance (governing second homes) since July 2006, a fact of which they were officially made aware in February 2007".
The committee, which reports on John Lyon's findings, said it was also concerned "about the length of time their arrangements had been allowed to continue" after the rules changed.
"This should have been addressed at an earlier stage," the committee says in its report.
'Let taxpayers down'
Commenting on the case, Matthew Elliott, chief executive of the TaxPayers' Alliance, said: "The Wintertons were wrong to continue this arrangement after they were told unequivocally that it was out of order.
"Those MPs who have been exposed for breaking the spirit of the expenses rules, if not always the letter, have clearly let taxpayers down, but the Parliamentary Authorities are also to blame.
"If the Fees Office had been tougher in applying the rules and if expense claims had been transparent and in the public domain, this sort of violation would never have happened."
An MPs' committee headed up by Speaker Michael Martin is reviewing the whole expenses system - in the wake of the case of Derek Conway, a Conservative MP who was reprimanded for the amount he had paid his son to work as a Parliamentary researcher.