The company, which is Europe's largest airline by passenger numbers, described itself as a “low fares, low carbon emissions,” airline in radio and TV adverts that ran in September last year.
Print adverts during the same campaign claimed that “Ryanair has the lowest carbon emissions of any major airline”.
But the ASA said in its ruling that those claims were not substantiated after questioning the evidence provided, and said adverts cannot run again in their current form.
"We told Ryanair to ensure that when making environmental claims they held adequate evidence to substantiate them and to ensure that the basis of those claims were made clear,' the ASA added.
Viewers — including a consultant in energy, transport, and sustainable development, according to ASA — complained to the watchdog that the adverts didn't stack up and challenged whether the claims could be substantiated.
After investigating, the watchdog took issue with some of the figures used by Ryanair and queried the definition of “a major airline” for the purposes of assessing emissions comparisons.
It stated that there was no commonly understood definition of what counts as a major airline, and that some airlines were not included in the comparison data Ryanair had pointed to as evidence.
Ryanair also argued that its green credentials come from having the newest energy efficient engines, a young fleet of aircraft, and from running 97% full on average — meaning that its carbon footprint per passenger average is lower than it would be if running emptier flights.
The airline provided a 2011 efficiency ranking which showed it ranked higher than other airlines on efficiency, however the ASA said that had "little value as substantiation for a comparison made in 2019".
The ASA concluded said that while customers probably know that travelling by plane is not a low CO2 option "in absolute terms", they could interpret the adverts’ claims to mean that travelling with Ryanair "would mean they were contributing less CO2 than travelling with other airlines", which could not be proved.
Ryanair said that it would comply with the ruling, the BBC reported, while adding in a statetment: "Ryanair is delighted with its latest environmental advertising campaign, which communicates a hugely important message for our customers."
The statement added: "The single most important thing any customer can do to halve their carbon footprint is switch to Ryanair."
Airlines have come under significant pressure from environmental campaigners and activists over the industry's carbon emissions, which constitute an estimated 2% of all the world's human-induced carbon emissions.
Between 2013 and 2018, worldwide CO2 emissions from commercial flights rose by 32%, according to a report published last year by the International Council of Clean Transportation.
Some airlines have responded by paying for carbon offsetting — meaning financially supporting projects that capture carbon. For example, EasyJet announced in November last year that it will offset all of its flights, reportedly making it the world's first "major airline" to operate net zero carbon flights.
In 2018, Ryanair launched a voluntary carbon offset scheme, which has so far raised €2.5 million (£2.1 million) according to the airline's website.
However, it doesn't seem ot have made a great deal of difference to its overall emissions output. Ryanair was named as the only airline included in an April 2019 list of Europe's top 10 polluters, according to data from the EU's Transport & Environment group.