Although email service Outlook appears to be resuming for some, the outage has so far lasted for more than 20 hours – in outage terms that is considered a long time.

Prof James Ferryman from the University of Reading’s department of computer science said the details the company has disclosed provide some clues.

“What Microsoft has told us is that the part of its servers that sets up secure connections is getting swamped, and once that happens the server just stops answering,” he said.

“The trouble is that this kind of failure snowballs.

“One server falls over, its work gets passed to the next one, and meanwhile huge numbers of phones and laptops keep trying to get back in.

“You can’t just turn it all back on at once. You have to bring it back bit by bit.”

The challenge is therefore not just fixing the fault, but managing the huge surge of devices trying to reconnect once services start coming back online.

Gergely Orosz, a former leading software engineer at several major tech companies including Skype and Uber, said this could be the “most disruptive” business email outage “of all time”.

He said it was still difficult to know just how many people have been affected by the disruption, but added he “cannot recall” an outage over business email ever taking this long to return to normal.

Australian cyber-security expert Troy Hunt told the BBC the outage could have far-reaching consequences.

“Microsoft is a massive part of the internet backbone and reminds us how critical email still is.

“Not only does this outage affect many people’s ability to work but also things like the ability to communicate about appointments, purchase things that need verification, create or access accounts and services,” he said.