The budget carrier said the move was part of its ongoing sustainability campaign, which it says includes more than €20 million investment in new, greener aircraft.

Less than 4% of Ryanair’s 150 million annual passengers currently contribute to the scheme, the proceeds of which go towards supporting carbon offset projects.

These include, over the past year, pledging €250,000 towards a reforestation project in the Monchique region of southern Portugal, devastated by the country’s 2018 forest fires.

Ryanair says it will aim to work with more offset schemes and partners in 2020 using the proceeds from its voluntary offset contributions.

The carrier claims to have the lowest emissions of any major EU airline at 66 grams of CO2 per passenger kilometre.

‘We’re committed to doing more’

Tom Fowler, the carrier’s first director of sustainability, who was appointed in December, said Ryanair paid out more than €600 million in environmental taxes last year, and contributed €1.5 million to carbon offsetting projects.

“We are committed to doing more,” said Fowler. “Our investments in new aircraft technology will enable us to carry more passengers, but with much lower fuel consumption and lower noise emissions.

“We are 60% of the way to being plastic free onboard by 2024, and our customers are now contributing more than €1.5 million per annum towards carbon offset projects. We thank them sincerely for their support for our ambitious sustainability programme.

“With this modest increase in voluntary contributions, we can do more in 2020, and we look forward to unveiling more 2020 environmental partners in the coming months.”

Ryanair has come under fire in the past for overstating its green credentials, or “greenwashing” after it was last year named among Europe’s top 10 carbon emitters in an EU report.

Earlier this month, ads claiming the carrier was “Europe’s lowest emissions airline” were ruled to be misleading by the Advertising Standards Authority (ASA).

The ASA said it had told Ryanair “to ensure that when making environmental claims, it held adequate evidence to substantiate them and to ensure that the basis of those claims were made clear”.

Ryanair said it was “disappointed and surprised” by the decision, and insisted it had fully complied with advertising regulations.