Channel 4 News breached broadcasting rules over its coverage of the Ellison Review into possible police corruption around the Stephen Lawrence case, according to communications watchdog Ofcom.
The Metropolitan Police complained about a section of the programme featuring a reporter interviewing members of the public about the issue.
It said the interviews were presented as being "chosen randomly", but in fact "the individuals were employees of an organisation (Livity4) for which the reporter had worked and which … had also worked for Channel 4".
The broadcaster told Ofcom it "accepted that this particular report fell below the normal standards of Channel 4 News" and said it was "caused by poor judgment by a junior reporter".
Ofcom said broadcasters had a "fundamental obligation … to ensure that audiences are not misled by the manner in which news is presented" and "breaches of this nature are amongst the most serious that can be committed by a broadcaster because they go to the heart of the relationship of trust between a broadcaster and its audience".
An Ofcom spokesman said: "Following a detailed investigation, Ofcom found Channel 4 breached broadcasting rules for not presenting its news coverage of the Ellison Review, which looked at possible corruption in the Metropolitan Police Service, with due accuracy.
"The reporter presented a series of vox pop-style interviews for a story in Brixton, with people chosen seemingly at random who gave views that were critical of the police. However, it did not make clear that these people were in fact pre-selected and had ties to the reporter."
A Channel 4 spokeswoman said: "Ofcom accepted that all the individuals interviewed in this brief report were expressing their own genuinely held opinions on camera, which reflected similarly held views by many others – and has ruled that the overall 20-minute item about the Ellison review was duly impartial.
"Whilst we do not agree that the audience was misled in any meaningful way, we accept that the reporter's methodology was flawed – it was an error of judgment and we broadcast an apology and clarification about this within a week on Channel 4 News."