Parliamentary officials were forced to lock MPs out of their own email accounts as they scrambled to minimise the damage from the incident.

The network affected is used by every MP including Theresa May, the Prime Minister, and her cabinet ministers for dealing with constituents.

Experts on Saturday night warned that politicians could be exposed to blackmail or face a heightened threat of terrorist attack if emails were successfully accessed.

MPs also apologised to their constituents and expressed concerns that sensitive and private information shared with them may have leaked.

Fears were raised by cyber specialists that “state actors” such as Russia, China or North Korea could be behind the attack - though Government sources said it was too early for conclusions.