US officials plan to axe the number of flights at 40 of the country's busiest airports - potentially resulting in thousands of cancelled flights as they try to ease pressure on air traffic controllers.

Control towers at airports have been grappling with absences of air traffic controllers who are calling out sick or taking side jobs as they work without pay during the federal government shutdown.

Altogether, hundreds of thousands of federal workers are not being paid because lawmakers have not been able to pass a budget. On Wednesday, the funding impasse became the longest shutdown in US history.

The Federal Aviation Administration (FAA) on Thursday formally announced the list of airports where flights will be cancelled. The cuts won't apply to international flights, but specific airlines may choose to also axe some of those flights, too, the FAA said.

Several of the largest US airlines have said customers will be allowed to adjust their flights or cancel them preemptively for free.

Here is what we know as the US prepares for the cuts, which will start officially on Friday.

Affected airports include the busiest in the world

Flights will be cut at Hartsfield-Jackson Atlanta International Airport, in Georgia. Frequently ranked as the busiest airport in the world, it saw more than 108 million passengers get on and off flights there in 2024.

Along with the Atlanta airport, Dallas Fort Worth International, Denver International, Chicago O'Hare, and Los Angeles International are the five busiest airports in the US. All of them are expected to see flights reduced.

The FAA said the cancellations will be gradual and start on Friday with a 4% reduction in flights. It will rise to 6% by 11 November and 8% by 13 November, before hitting a full 10% by 14 November.

1. Anchorage International (ANC)
2. Hartsfield-Jackson Atlanta International (ATL)
3. Boston Logan International (BOS)
4. Baltimore/Washington International (BWI)
5. Charlotte Douglas International (CLT)
6. Cincinnati/Northern Kentucky International (CVG)
7. Dallas Love Field (DAL)
8. Ronald Reagan Washington National (DCA)
9. Denver International in Colorado (DEN)
10. Dallas/Fort Worth International (DFW)
11. Detroit Metropolitan Wayne County (DTW)
12. Newark Liberty International (EWR)
13. Fort Lauderdale/Hollywood International (FLL)
14. Honolulu International (HNL)
15. Houston Hobby (HOU)
16. Washington Dulles International (IAD)
17. George Bush Houston Intercontinental (IAH)
18. Indianapolis International (IND)
19. New York John F Kennedy International (JFK)
20. Las Vegas Harry Reid International (LAS)
21. Los Angeles International (LAX)
22. New York LaGuardia (LGA)
23. Orlando International (MCO)
24. Chicago Midway (MDW)
25. Memphis International (MEM)
26. Miami International (MIA)
27. Minneapolis/St Paul International (MSP)
28. Oakland International (OAK)
29. Ontario International in California (ONT)
30. Chicago O'Hare International (ORD)
31. Portland International (PDX)
32. Philadelphia International (PHL)
33. Phoenix Sky Harbor International (PHX)
34. San Diego International (SAN)
35. Louisville International (SDF)
36. Seattle/Tacoma International (SEA)
37. San Francisco International (SFO)
38. Salt Lake City International (SLC)
39. Teterboro in New Jersey (TEB)
40. Tampa International (TPA)

Airlines offering refunds to customers

Major airlines have raced to assure customers that they will not be caught for the costs of cancelled flights.

In a memo to staff that was shared with the BBC, United Airlines CEO Scott Kirby said any customer can get a refund on a flight, even if it isn't being cancelled, including customers with non-refundable tickets and basic economy tickets. United, the largest airline in the country, says that, in addition to waiving fees, it will also waive any fare differences on a rebooked flight.

Delta Airlines has implemented a similar policy. allowing customer change, cancel or receive refunds for their flights, including those flying basic economy, without penalty. If the rebooked flight occurs on or before 16 November, any difference in fare will be waived, the company said in a statement.

American Airlines will allow customers to change a flight or request a refund, without penalty, whether or not their specific flights have been cancelled. It does not mention if customers will have to pay a difference in fare on rebooked flights.

Southwest Airlines says that if customers' flights are cancelled, they can rebook or request a refund. For flights between 7 November and 12 November that have not been cancelled, customers can still rebook with no fee and without having to pay a fare difference, or they can request a refund.

Frontier Airlines will let customers change or cancel their flights without a fee, but they will be required to pay any difference in fare on a rebooked flight.

The BBC has also reached out to Alaska Airlines and JetBlue.