Britain's Advertising Standards Authority (ASA) has upheld complaints against two of four Ryanair advertisements which criticised Gordon Brown over air passenger duty.
The airline used pictures of the then Chancellor in press adverts which complained about the increased tax rate.
One advert said Mr Brown had "fleeced" holidaymakers in an air passenger tax "deception", adding: "£1 billion just disappears into greedy Gordon's pockets."
The advert continued: "Not a penny spent on the environment, aviation accounts for just 2 per cent of CO2 emissions and yet he pulled it off - unbelievable."
Four Ryanair press adverts each featuring Mr Brown drew 48 complaints from readers. ASA today upheld objections, saying the airline had breached advertising rules relating to truthfulness.
Ryanair's claim that aviation made up 2 per cent of CO2 emissions was based on global carbon dioxide emissions, the industry watchdog said.
Treasury figures for emissions from domestic UK flights and international flights leaving the UK put CO2 emissions at 5.5 per cent of the UK's total carbon dioxide output, the ASA found.
The watchdog said: "We considered that, because Ryanair had failed to make the basis of the 2 per cent figure quoted in the ad sufficiently clear, it was likely to mislead."
Referring to air passenger duty, one of the Ryanair adverts included the claim "...not a penny spent on the environment".
That claim was misleading because the airline could not prove the Government had not spent — or did not intend to spend — any revenue from air passenger taxes in this way, the ASA said.
The watchdog told Ryanair not to repeat the misleading claims in future adverts. Responding to the inquiry, Ryanair said its 2 per cent figure had come from the United Nations' inter-governmental panel on climate change.
The airline said it used a global figure for the aviation industry's CO2 emissions because the issue was a global one. Ryanair insisted the British government had not earmarked any money from air passenger duties for environmental projects. The airline received no reply when it wrote to Mr Brown about the issue, it said.
The ASA rejected complaints from readers who challenged Ryanair's claim that "UK tourists will pay £1 billion more in taxes".
Ryanair criticised the ASA's decision and accused the watchdog of trying to "cover up" a "travel tax scam".
Responding to the two points on which it was found in breach of the advertising code, the airline said it would defy the ASA's ruling by continuing to use them in future adverts. "For as long as this travel tax scam continues, Ryanair will continue to use both of these factually accurate statements in its advertising," Ryanair said.