The voluntary aviation disputes scheme no longer counts Ryanair as a member.
The move comes after the budget airline received more than 14,000 complaints and was forced to pay £3.6 million in handling fees and compensation last year.
Only 87 of those have been resolved, leaving customers frustrated and out of pocket.
Which? calling for the aviation sector to have a single mandatory resolution scheme that handles complaints swiftly and “has real teeth”.
The consumer body said Ryanair’s decision to leave Aviation ADR – one of two non-mandatory complaints bodies – at the end of November last year is set to potentially save Ryanair millions, but could leave passengers struggling to get compensation.
While both have been authorised to handle escalated passenger complaints since 2016, neither is mandatory.
Ryanair was contacted by The Scotsman, but did not comment.