Ofcom has ruled that ITV misled viewers by airing footage claimed to have been shot by the IRA, which was actually material taken from a video game.
A total of 26 people alerted the regulator, raising concerns over the footage broadcast in Exposure: Gaddafi and the IRA, in September.
ITV apologised after the issue came to light, saying it was "an unfortunate case of human error".
Ofcom said it was a "significant breach of audience trust".
The current affairs programme was investigating the financial and military links between the former Libyan leader, Colonel Gaddafi, and the IRA.
During the documentary, footage labelled "IRA Film 1988" was shown, described as film shot by the IRA of its members attempting to shoot down a British Army helicopter in June 1988.
However, the pictures were actually taken from a game called Arma 2.
ITV said the programme had intended to use footage of "a genuine incident" which had been included in an episode of The Cook Report.
While trying to source "a better version" of the footage, the programme director viewed footage from the internet which "he mistakenly believed... to be a fuller version".
ITV said that "regrettably" the internet footage was not cross-checked and verified by the production staff as being The Cook Report footage.
In another instance, footage of police clashing with rioters in Northern Ireland was described as having taken place in July 2011. But viewers complained to Ofcom that due to the type of police riot vehicles shown, the footage must have been of an earlier riot.
ITV said although the incident referred to did happen, it admitted the footage was not from July 2011.
It said the programme's director had requested the film from a local historian who had supplied footage to broadcasters in the past and was considered a trustworthy source, however due to a "miscommunication" between the two parties, "the discrepancy... was not discovered".
ITV said the documentary had included footage intended to portray two real events and apologised that in each case "the wrong footage" was used, adding "mistakes were the result of human error and not an intention to mislead viewers".
Finding ITV in breach of the broadcasting code, Ofcom said it was "greatly concerned" the broadcaster failed to authenticate the two pieces of footage.
It said there were "significant and easily identifiable differences" between The Cook Report footage and the footage taken from the video game and was therefore "very surprised that the programme makers believed the footage of the helicopter attack was authentic".
The regulator added it was also "not sufficient for a broadcaster or programme maker to rely on footage provided by a third party source, on the basis that that source had previously supplied other broadcasters with archive footage".
"We take into account that ITV: apologised; removed the programme from its catch-up video-on-demand service; and has now put in place various changes to its compliance procedures to ensure such incidents do not happen in future," Ofcom said.
"However, the viewers of this serious current affairs programme were misled as to the nature of the material they were watching."
Meanwhile, Ofcom has said it will not investigate complaints about BBC programme Frozen Planet, after it aired footage of newborn polar bear cubs filmed in an animal park, rather than in the wild.
Ofcom said five people complained the show was misleading as they had assumed the cubs were born and filmed in the Arctic.
The BBC said the filming was "standard practice" for natural history shows.
The regulator said "after careful assessment", it decided not to pursue the programme as it "did not raise issues warranting investigation".