The hacker compromised the firm’s global email server through an “administrator’s account” that, in theory, gave them privileged, unrestricted “access to all areas”.

The account required only a single password and did not have “two-step“ verification, sources said.

Emails to and from Deloitte’s 244,000 staff were stored in the Azure cloud service, which was provided by Microsoft. The hackers had potential access to usernames, passwords, IP addresses, architectural diagrams for businesses and health information. Some emails had attachments with sensitive security and design details.