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 massive global network outage.

The US-based carrier accused the cybersecurity company of “negligence”, saying it was forced to cancel thousands of flights because of the outage 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 its huge number of customers.

Microsoft estimated that 8.5 million Windows devices around the world were disabled as a result.

Delta Airlines’ services were impacted for days after the outage, even after other airlines appeared to have recovered. Delta 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 against the two companies.

Both CrowdStrike and Microsoft have rejected the claim that they are responsible for the disruptions at Delta.

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it would fight back, and added that its preliminary review shows Delta, unlike its competitors, was operating with an outdated IT infrastructure.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

In response, a CrowdStrike spokesperson accused Delta of pushing “a misleading narrative”.

Delta is facing its own legal challenges after the outage, after a lawsuit was filed against it 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 passengers to sign waivers releasing Delta of all legal claims.

Many airlines rely on Microsoft’s Office365 for scheduling. The CrowdStrike outage had crashed those systems, forcing them to resort to 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.