A cyber attack on Westminster compromised almost 100 email accounts, parliamentary officials have confirmed.

An unprecedented attack by hackers seeking to break into MPs’ inboxes utilising a “brute force” assault which lasted for more than 12 hours on Friday prompted fears that senior politicians could become the targets of blackmail.

But the “sustained and determined” offensive is now believed to have only compromised up to 90 email accounts with less than one per cent of Parliament’s 9,000 users directly impacted.

An investigation is now underway to determine whether any data has been lost.

Unknown 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.