BBC rules on impartiality were broken during a report on the Ten O'Clock News, the corporation's governors said.
Guto Harri is now the BBC's North American correspondent
The complaint surrounded Guto Harri's report about the night's edition of Question Time, a month before the General Election last year.
Governors said rules were broken when Harri said the then Conservative leader was booed, but did not mention the same thing had happened to Tony Blair.
The omission breached BBC guidelines on accuracy and impartiality, they ruled.
However, they also said that overall the report gave a "fair and balanced" account of the event, and did not uphold the broader complaint of evidence of political bias.
The Question Time broadcast on 28 April 2005 featured interviews with Charles Kennedy, then Liberal Democrat leader, as well as former Conservative leader Michael Howard and Mr Blair.
The three main party leaders had been questioned by a studio audience with Mr Blair and Mr Howard booed when they walked on stage.
The Governors' Programme Complaints Committee concluded: "Blair's Question Time entrance was greeted by pronounced booing and this did not come across in the subsequent news report.
"Given that the report referred to booing during Michael Howard's entrance, it ought also to have referred to booing during Tony Blair's entrance," it said.
"Failure to do so gave an inaccurate and partial impression of the reception Tony Blair received.
"The committee concluded therefore that, in the context of the report, the failure to refer to booing at Tony Blair's entrance had been unfortunate and breached the guidelines on accuracy and impartiality.
"This element of the complaint was upheld."
A Conservative Party spokesman said: "There were a number of occasions during the General Election when the BBC's coverage fell below its normally high standards.
"This ruling will serve as a reminder to the BBC that they must meet the highest standards of accuracy and impartiality."