A Harrow MP has been ordered to repay more than £1,000 in travel expenses after claiming more than any others - for driving around his constituency.
Almost all of the mileage claims submitted by Mr Blackman, 58, over a 19-month period, had been inflated.
The report concluded that "mileage claims submitted by Mr Blackman are, in almost every instance, not accurate and greater than the distance travelled".
IPSA had been warning the MP about his mileage claims since 2011 and the report said they were "around twice the average" claimed by other MPs.
An earlier investigation into the Tory MP’s travel expenses, published in 2012, compared the mileage he claimed for driving around his constituency with data from Google Maps.
The report said Mr Blackman appeared to be vastly over-estimating the distance he had travelled, and suggested "a pattern of over-claiming".
The compliance officer had a discussion with Mr Blackman and no further action was taken at the time.
But an audit of his claims during the 2013/14 financial year showed the backbencher’s mileage claims were higher than any other MP, and a formal investigation was launched.
The probe, published on Wednesday (February 4), found Mr Blackman had submitted a total of 734 inaccurate claims and ordered him to repay £1,006.20. He rejects the findings of the investigation.
A statement on the MP’s website claimed the compliance officer had based estimates of the actual distance travelled to street surgeries on "other people’s Twitter accounts".
He said: “It states in the IPSA guidelines that the most cost effective route should be taken. The compliance officer believes that the most direct route was not taken. I supplied detailed maps for the routes I routinely took which the compliance officer ignored.
“I am concerned that the compliance officer is not interpreting the rules that exist but rules that he believes should exist.”
Mr Blackman has 15 days to appeal the watchdog’s findings.