A “significant” number of the 8.5m devices affected by last week’s global IT outage are back online, according to the cybersecurity company at the centre of the incident.

CrowdStrike said it was also testing a technique to reboot systems more rapidly, amid warnings from experts that a full recovery from Friday’s IT failure could take weeks.

On Friday, thousands of flights were cancelled, broadcasters were forced off air, healthcare appointments disrupted and millions of PCs failed to start after a CrowdStrike software update inadvertently crippled devices using the Microsoft Windows operating system.

CrowdStrike wrote in a social media update that it had made progress in fixing the consequences of a glitch that, according to one expert, had caused “the largest IT outage in history”.

“Of the approximately 8.5 million Windows devices that were impacted, a significant number are back online and operational,” the US company said.

CrowdStrike added that it was testing a new method to “accelerate impacted system remediation” and working to get companies and organisations a means of accessing that technique.

CrowdStrike’s chief security officer, Shawn Henry, issued a statement on LinkedIn further apologising for the outage on Monday. He called the previous 48 hours the most challenging time in the dozen years he had spent at the company, saying that the company rapidly lost customer confidence it had spent ages earning.

“We let down the very people we committed to protect, and to say we’re devastated is a huge understatement,” Henry said in the post.

On Sunday, Australia’s home affairs minister said CrowdStrike was “close to rolling out an automatic fix to the issue with their update, as is Microsoft”.

On Friday, experts had warned that repairs to affected PCs would have to be carried out manually, potentially prolonging the recovery.

More than 9,600 flights have been cancelled worldwide since Friday, according to flight data company OAG, with the Atlanta-based Delta Air Lines accounting for nearly half that total, while in the UK 45 flights were cancelled on Saturday. A further 1,800 flights have been cancelled across the airline industry on Monday, according to aviation analysis firm Cirium.

Delta’s chief executive, Ed Bastian, said the problem had affected a critical application in its IT system. “In particular, one of our crew tracking-related tools was affected and unable to effectively process the unprecedented number of changes triggered by the system shutdown,” he told customers.

Europe’s largest airline, Ryanair, said it had cancelled 400 flights this weekend, mainly due to fallout from the IT outage.

In the UK, NHS England warned of delays as healthcare services recovered from the outage. It said patients with appointments this week should continue to attend unless told not to.

The British Medical Association said on Sunday that normal GP service could not be resumed immediately after the IT problems caused a considerable backlog.

An NHS spokesperson said: “Systems are now back online … Thanks to the hard work of NHS staff throughout this incident we are hoping to keep further disruption to a minimum, however there still may be some delays as services recover, particularly with GPs needing to rebook appointments, so please bear with us.”

Pharmacy services were expected to be “slower than usual” in the UK on Monday as the recovery continued.

Nick Kaye, the chair of the National Pharmacy Association, said: “As pharmacists recover from last week’s IT outage and catch up on the backlog of prescriptions, we expect service in some community pharmacies to be slower than usual today.”

Kaye asked customers to “be patient” with their local pharmacy teams.