The campaign, which was created in-house by Ryanair, was produced as a response to a hike in air passenger tax imposed by the then Chancellor of the Exchequer to pay for green initiatives.
The campaign included four press ads featuring Brown's head imposed on a cartoon robber's body with straplines such as 'The Great Plane Robber' and 'Greedy Gordon wins Oscar...for best stick up'.
The ads ran in The Daily Telegraph and The Times, provoking 48 complaints. Some of the complainants, including AirportWatch, challenged the claim made in two of the ads that: "Aviation accounts for just 2% of CO2 emissions".
Ten complainants objected to the use of an image of the Pope in one ad on the basis it was offensive to Christians; the ad also ran in newspapers on Ash Wednesday.
The Advertising Standards Authority upheld complaints directed at Ryanair's claims about greenhouse gases, on the basis the ads did not clarify its figure of 2%.
It said: "We noted there were many different figures available for aviations contribution to climate change, including global emissions or individual countries' emissions, as well as greenhouse gas emissions...we therefore considered that, to avoid confusion, it was important that the basis of the figure in an ad was made clear."
However, the ASA dismissed complaints about the use of an image of the Pope, saying readers were likely to interpret the ad as mocking Gordon Brown, rather than denigrating the Pope or Catholic religion.
The ASA also rapped Ryanair for providing insufficient evidence to prove the government had not spent any of the money from the tax increase on the environment.