Sir George Russell agreed that the interview had been "a little too friendly and relaxed" in a letter to the Labour Party yesterday, although the ITC said that Independent Television News, which produces the bulletin, had not breached the watchdog's code.
The interview was given top billing despite the TWA crash in New York in which 230 people died. Mr Major spoke to Trevor McDonald, the news anchorman, for seven minutes.
At one point, Mr McDonald, who was accused of grovelling, said to the Prime Minister: "I have been reading some of the interviews you have been giving to newspapers recently and what comes over is the extraordinary dedication you have for this job."
He was also attacked by Labour for not probing Mr Major over his record on the economy, tax cuts, Tory splits over Europe or his hopes of re-election. A senior Labour source claimed: "You would not have got an interview as fawning as that on Albanian TV."
In the letter to David Hill, Labour's chief media spokes-man, Sir George said the questions in which Mr McDonald referred to Mr Major's "extraordinary dedication" could be viewed as "inappropriate". He wrote: "We believe that setting exactly the right tone in such situations is something ITN may wish to consider further for the future."
Mr Hill had written to the ITC following the broadcast on 18 July urging an investigation into the Labour Party's formal complaint that News at Ten had "breached the impartiality section of the ITC programme code". He claimed the questioning was not "dispassionate" or "even-handed".
A Labour spokesman said yesterday: "We are delighted that the ITC has agreed with Labour that the tone of Trevor McDonald's interview with the Prime Minister was inappropriate, and are pleased that the ITC has given a commitment to monitor carefully the impartiality of broadcasters in the run-up to the election."
ITN said it welcomed the statement from the ITC that News at Ten had not breached the code on impartiality, and insisted that Mr McDonald was merely acting in a characteristically polite manner.
The suggestion that a deal was made with Downing Street, guaranteeing Mr Major the lead slot in return for the interview, has already been dismissed as "completely untrue" by Stewart Purvis, ITN's chief executive.