How does one decide which airlines are the world's worst?

Poor safety or regulatory standards will be the most important factor for many people, and there are currently more than 100 airlines - most of which you probably haven't heard of - banned from EU airspace, or facing operational restrictions, as they don't come up to scratch. They include every airline from Afghanistan, Kyrgyzstan, Liberia, Mozambique, Sierra Leone and Sudan, as well as dozens from Indonesia.

The website AirlineRatings.com, meanwhile, awards carriers a star rating based on safety standards. Just 10 muster only a single star: Batik Air, Bluewing Airlines, Citilink, Kal-Star aviation, Lion Air, Sriwijaya Air, TransNusa, Trigana Air Service, Wings Air, Xpress Air.

Lion Air is one of 10 carriers with a single star for safety, according to AirlineRatings.com Credit: 2014 Bloomberg/Bloomberg

Skytrax, another aviation ratings website, judges carriers according on the quality of their product and services.

Nine carriers (ANA, Asiana, Cathay Pacific, Etihad, EVA Air, Garuda Indonesia, Hainan Airlines, Qatar Airways and Singapore Airlines) can boast full marks. There are 40 with four stars, including Virgin, Lufthansa, BA, Air France and - surprisingly, perhaps - Emirates, which was voted top long-haul airline by Telegraph Travel readers earlier this year. A total of 107 airlines have three stars, including Ryanair, easyJet, United, WOW, Wizz, Norwegian, Thomas Cook, Monarch, Jet2 and American Airlines, the world's largest carrier.

But 21 airlines fall below that standard, according to Skytrax. Here they are (in alphabetical order):

1. Bahamasair

Based: Nassau

Founded: 1973

Fleet: 8

Destinations: 32

Slogan: "We don't just fly there, we live there"

Bahamasair, founded in 1973

2. Biman Bangladesh Airlines

Based: Dhaka

Founded: 1972

Fleet: 12

Destinations: 18

3. Bulgaria Air

Based: Sofia

Founded: 2002

Fleet: 10

Destinations: 26

4. China United Airlines

Based: Beijing

Founded: 1986

Fleet: 36

Destinations: 20

5. Cubana Airlines

Based: Havana

Founded: 1930

Fleet: 20

Destinations: 20

Slogan: "Cuba's Gateway to the World"

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6. Iran Air

Based: Tehran

Founded: 1944

Fleet: 26

Destinations: 58

Slogan: "Our Mission is Your Safety" & "We Take You There, We Take You Back"

7. Lion Air

Based: Jakarta

Founded: 1999

Fleet: 111

Destinations: 126

Slogan: "We Make People Fly"

8. Mahan Air

Based: Tehran

Founded: 1992

Fleet: 56

Destinations: 61

Slogan: "The Spirit of Excellence"

Mahan Air's rating doesn't match its slogan Credit: 2015 Anadolu Agency/Anadolu Agency

9. Nepal Airlines

Based: Kathmandu

Founded: 1958

Fleet: 9

Destinations: 33

10. Onur Air

Based: Istanbul

Founded: 1992

Fleet: 30

Destinations: 30

Slogan: "Fly Turkey, Fly!"

11. Pegasus Airlines

Based: Istanbul

Founded: 1990

Fleet: 75

Destinations: 103

Slogan: "We Didn't Start Aviation In Turkey, But We Transformed It"

Pegasus is one of the better known carriers on the list Credit: 2012 AFP/PASCAL PAVANI

12. Rossiya Airlines

Based: St Petersburg

Founded: 1932

Fleet: 61

Destinations: 58

13. SmartWings

Based: Prague

Founded: 2004

Fleet: 18

Destinations: 27

14. Spirit Airlines

Based: Miramar, Florida

Founded: 1980

Fleet: 95

Destinations: 59

Slogan: "Less Money, More Go"

Spirit is the only North American airline to receive fewer than three stars Credit: GETTY

15. Sudan Airways

Based: Khartoum

Founded: 1946

Fleet: 4

Destinations: 15

16. SyrianAir

Based: Damascus

Founded: 1946

Fleet: 12

Destinations: 14

17. Tajik Air

Based: Dushanbe

Founded: 1923

Fleet: 14

Destinations: 19

18. Turkmenistan Airlines

Based: Ashgabat

Founded: 1992

Fleet: 33

Destinations: 23

Slogan: "Symbol of Hospitality"

19. Ukraine International Airlines

Based: Kiev

Founded: 1992

Fleet: 38

Destinations: 80

Ukraine International was also placed on the bottom shelf Credit: ALAMY

20. Yemenia

Based: Sana'a

Founded: 1962

Fleet: 6

Destinations: 29

The worst airline of all? According to Skytrax, it's Koryo Air, based in Pyongyang, North Korea. While all the others listed here received two stars, it was handed just one.

Koryo is the world's worst airline, according to Skytrax Credit: 2013 AFP/ED JONES

Previous passengers have described "perfunctory" emergency procedures, fizzy drinks that are "difficult to identify" and "uncomfortably loud" engine noise, while photographers on board its oldest aircraft have captured cockpits without digital screens and seats that fold forward. Furthermore, the in-flight entertainment usually consists of patriotic odes to the leader and indecipherable North Korean cartoons.

But not everyone agrees with the scathing verdicts. “It’s a bit of a giggle, actually,” Simon Cockerell of Koryo Tours, an independent Beijing-based travel company that specialised in trips to North Korea, told Associated Press earlier this year. “It’s not Emirates. It’s not quite the flying experience people are used to… [but] they are clearly not the world’s worst airline.”

Indeed, some of the passenger reviews on Skytrax’s own website are in fact complimentary.

“Service was perfectly passable,” wrote one UK user earlier this year.

“The burger was quite tasty and I got a beer with it,” said another.

Baltia might just be worse than even Koryo

Even worse than Koryo might just be Baltia. Never heard of it? That's because it has never flown once in its 27-year history. Founded in 1989 with the aim of offering a “high-quality three-class passenger service”, it is still awaiting its operational approval from the US Federal Aviation Administration. The airline is said to have failed the FAA evaluation seven times, apparently due to a problem with the deployment of its aircraft slides. Take that Koryo.