US Transportation Secretary Sean Duffy has warned that air travel capacity will be cut by up to 10% at 40 major airports over the coming days, if the government shutdown continues.

The decision, which is expected to affect domestic flights only, was made because air traffic controllers had been reporting fatigue, the head of the Federal Aviation Administration (FAA) said at a briefing alongside Duffy.

Some 1.4 million federal workers, from air traffic controllers to park wardens, are working without pay – or are on forced leave – because the US Congress has not agreed a federal funding budget.

Unions say many employees are becoming ill with stress or are being forced into taking second jobs.

On Wednesday, the impasse became the longest shutdown in US history.

“It is unusual,” said FAA chief Bryan Bedford of the planned flight reductions, “just as the shutdown is unusual, just as the fact that our controllers haven’t been paid for a month is unusual”.

The flight reductions will be gradual, starting at 4% of domestic flights on Friday, then rising to 5% on Saturday and 6% on Sunday, before hitting the full 10% next week, the Reuters news agency reported, citing four unnamed sources.

The names of the affected airports – all high-traffic locations – would be released on Thursday, the officials said.

US media including the BBC’s partner CBS News reported that the list would include some of the nation’s busiest hubs, including Hartsfield-Jackson Atlanta International, Dallas/Fort Worth International, Denver International, Chicago O’Hare International and Los Angeles International.

The cancellations could affect between 3,500 and 4,000 flights per day. US media also reported that international flights would not be affected.

“We are seeing pressures build in a way that we don’t feel – if we allow it to go unchecked – will allow us to continue to tell the public that we operate the safest airline system in the world,” Bedford said during Wednesday’s briefing.

Duffy insisted that air travel was still safe, and the decision to cancel the flights was being made to maintain safety and efficiency.

If the shutdown continued and added more pressure to the system, additional restrictive measures may be required, Bedford said.

In a statement, American Airlines, the second-largest carrier in North America, said it was awaiting additional information from the FAA so it could determine which flights will be scrapped, but that “we expect the vast majority of our customers’ travel will be unaffected”.

A spokesperson for Southwest Airlines, another major carrier, said in a statement that the company was still evaluating how the flight restrictions would affect its services, and that it would let customers know as soon as possible.

“We continue to urge Congress to immediately resolve its impasse and restore the National Airspace System to its full capacity,” the spokesperson added.

Delta Airlines declined to comment. The BBC has also asked for comment from other major US airlines.

Once government funds ran out on 1 October, most federal workers were sent home and told they would be paid once the government reopened. Those deemed essential, like controllers, though, had to keep doing their jobs without pay.

Almost immediately after the shutdown started, airports began feeling the effects. Some had to ground flights for hours after air traffic controllers called out sick, while others relied on controllers from other airports.

Duffy warned earlier this week that the flight cancellations could be coming, as half of the country’s 30 major airports experienced staff shortages.

Nick Daniels, the president of the labour union representing more than 20,000 aviation workers, put the situation into stark terms on Wednesday.

“Air traffic controllers are texting ‘I don’t even have enough money to put gas in my car to come to work,'” he told CNN.

“We base what we do day in and day out on predictability,” he said. “Right now there is no predictability.”

Speaking to NPR, one controller suggested that the situation might be reaching “tipping point” for them and their colleagues. “This is kind of about the point in the last shutdown where people just started getting fed up with it,” they said.

Another controller told the same broadcaster: “It does degrade that margin of safety if a bunch of people are sick and not at work and I’m having to do their jobs along with my own.”

Writing recently for MSNBC, another controller, a single father, said he was working for food delivery service DoorDash after his daily air traffic shift ended, and was “sleeping only two hours most nights”.

“Like many families, we didn’t plan for a shutdown,” he wrote. “Yet the bills don’t stop.”

Duffy previously said there was a risk that came with air traffic controllers taking on additional jobs during the shutdown, and had threatened to fire controllers who did not come to work.

“They have to make a decision, do I go to work and not get a paycheque and not put food on the table? Or do I drive for Uber or DoorDash or wait tables?” Duffy said on Sunday. (BBC News)