A Channel 4 investigation examining Ken Livingstone's eight-year tenure as London mayor has been cleared by the broadcasting watchdog.
Ofcom received 12 complaints about the Dispatches: The Court of Ken documentary, fronted by New Statesman political editor Martin Bright, which was broadcast in January.
The programme questioned whether the mayor was best serving the needs of London, and accused Livingstone of electoral malpractice and cronyism.
The documentary also looked at the issues surrounding the London congestion charge and an "oil for transport" deal struck between the mayor's office and the Venezuelan president Hugo Chavez.
The complainants argued that the allegations made against Livingstone were unsubstantiated and claimed the programme was not presented with due impartiality.
Ofcom said Livingstone himself was not one of the people to have lodged a complaint.
At the time of broadcast, a spokesman for Livingstone described the claims made as "grossly biased" and accused Channel 4 of "smearing" the mayor's reputation in the run-up to the election.
In its ruling today, the regulator found that the programme included a number of viewpoints, both supportive and critical, about the congestion charge.
It also concluded that the Venezuelan deal was politically controversial and was presented with due impartiality.
The watchdog did not look into claims of unfair treatment of Livingstone, because it would only be able to do so if the mayor himself were to have complained.
In its conclusion, Ofcom said: "Investigative journalism plays an essential role in public service broadcasting and is clearly in the public interest.
"Ofcom considers it of paramount importance that broadcasters, such as Channel 4, continue to explore controversial subject matter."