Ryanair has been named the filthiest flight operator in the UK by passengers, according to a Which? Travel survey.
In the survey of almost 8,000 people, fewer than half of passengers (42%) scored the Irish budget airline as good for cleanliness.
In contrast, the survey found that on average, eight in 10 (81%) of passengers rated hygiene positively across 42 airlines.
But Ryanair customers were unhappy, with a quarter (24%) claiming cleanliness on flights was poor.
A Which? Investigator who used an ultraviolet light to examine the cleanliness of a Ryanair flight was greeted by greasy tray tables, soiled headrests and dusty window sills upon boarding.
Meanwhile, the ultraviolet light showed up stains on the tray tables that could not be seen by the naked eye.
Rory Boland, Which? Travel editor, said: "Faster and faster turnarounds are one thing but it is unacceptable for some airlines to be cutting corners when it comes to cleaning out their cabins properly – no matter how cheap the airline ticket is.
"There are steps you can take; either choose your next flight on an airline that has a good track record for cleanliness or equip yourself with some antibacterial wipes.
"If you're flying Ryanair though, a biohazard suit might be more appropriate."
While Ryanair was not the only airline to be criticized for its hygiene, it was significantly worse than the other airlines included in the survey.
Many WizzAir customers were also less than impressed with their cleaning, with only six in 10 (62%) finding the cleanliness to be good.
The scores were almost better for those who chose to fly with Vueling or Iberia, with 63% of passengers being satisfied with the cleanliness.
Around two-thirds (68%) of Easyjet passengers rated the cleanliness as "good", "very good" or "excellent".
British Airways was also in passengers' good books with three in four (78%) being pleased with the airline's hygiene.
Nearly all of the passengers who flew with Air New Zealand (97%), Singapore Airlines (96%), Emirates (95%), Qatar Airways (95%), Cathay Pacific (94%) and Swiss (94%) told Which ? that they found the cleanliness on board to be good.
Last year, research conducted by Marketplace, a Canadian TV show, revealed the grim reality of the various types of bacteria, mold and yeast that are on planes.
After analyzing more than 100 samples collected on 18 domestic Canadian flights, scientists discovered that headrests were the most contaminated place on a plane.
Offering tips to passengers on how to protect themselves, Which? advised washing hands before eating, carrying a hand sanitizer gel and avoiding touching your eyes, mouth or nose while on board.
Sky News has reached out to Ryanair for comment.