Delta Airlines has expressed frustration with CrowdStrike in a new letter on Thursday, as the two companies continue to trade jabs after last month’s global network failure.

The US-based carrier accused the cybersecurity company of “negligence”, saying it was forced to cancel thousands of flights because and had lost at least $500m (£392m) as a result.

CrowdStrike had denied it was solely responsible for Delta’s flight disruptions, which it said continued after other carriers came back online.

Delta has since been hit by a class-action lawsuit filed on behalf of affected passengers.

The global glitch originated from CrowdStrike on 19 July, after it had sent out a corrupted software update to customers.

Microsoft estimated it disabled 8.5 million Windows devices around the world.

Delta Airlines’ services were affected for days after, even as other airlines appeared to have recovered. It cancelled around 7,000 flights over five days until 24 July, and is now being investigated by the US Department of Transportation over the disruptions.

The airline has since blamed CrowdStrike and Microsoft for the disruptions, and has threatened legal action. Both firms have rejected the claim that they were responsible.

Delta’s CEO Ed Bastian wrote in a filing with the US Securities and Exchange Commission on Thursday that what happened was “unacceptable”.

“Our customers and employees deserve better,” Mr Bastian wrote, adding that the technology meltdown affected 1.3 million of Delta’s customers.

CrowdStrike said on Sunday that it would defend itself “aggressively” should Delta take legal action against it.

Microsoft also said it would fight back, and added that its preliminary review shows Delta, unlike its competitors, was operating with an outdated IT infrastructure.

In response, David Boies, a lawyer for Delta, wrote in a letter to CrowdStrike on Thursday that “there is no basis – none – to suggest that Delta was in any way responsible for the faulty software that crashed systems around the world”.

He added that Delta Airlines had invested billions of dollars in its technology, and said it struggled to restore operations because of its reliance on Microsoft and CrowdStrike.

CrowdStrike accused Delta of pushing “a misleading narrative”.

A lawsuit has also been filed against Delta on behalf of passengers whose flights were cancelled.

The legal action stated that “no other US airline had cancelled one-tenth as many flights”.

It also claimed that Delta failed to properly compensate passengers, and that it had asked themto sign waivers releasing Delta of all legal claims.

Many airlines rely on Microsoft’s Office365 for scheduling. The CrowdStrike failure crashed those systems, meaning firms had to use manual scheduling.

CrowdStrike has since been sued by its shareholders, who accused the company of making “false and misleading” statements about its software testing. CrowdStrike has denied the allegations.