Iran was responsible for a cyberattack on Parliament that hit dozens of MPs, including Theresa May, Whitehall sources have told The Daily Telegraph.

The unprecedented “brute force” cyber attack in June lasted more than 12 hours and compromised around 90 email accounts.

It was initially thought that Russia was behind the attack, but intelligence officials have now concluded Iran is responsible. It would be Tehran's first significant cyberattack on the UK.

In the attack, hackers repeatedly probed "weak" passwords of politicians and aides, forcing parliamentary officials to lock MPs out of their own email accounts as they scrambled to minimise the damage from the incident.