The Guardian reported that MPs were unable to gain access to their email on devices outside Westminster after the attack commenced.
An email sent to those who were affected said: "“Earlier this morning, we discovered unusual activity and evidence of an attempted cyber attack on our computer network. Closer investigation by our team confirmed that hackers were carrying out a sustained and determined attack on all parliamentary user accounts in an attempt to identify weak passwords.
“These attempts specifically were trying to gain access to our emails. We have been working closely with the National Cyber Security Centre to identify the method of the attack and have made changes to prevent the attackers gaining access, however our investigation continues.”
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Statement regarding cyber incident. pic.twitter.com/fAbDkAfdbj
— Commons Press Office (@HoCPress) June 24, 2017
International trade secretary Liam Fox told the BBC: "We know that there are regular attacks by hackers attempting to get passwords.
"We have seen reports in the last few days of even Cabinet ministers’ passwords being for sale online.
"We know that our public services are attacked, so it is not at all surprising that there should be an attempt to hack into parliamentary emails.”
Britain's National Crime Agency tweeted that it was looking into the matter with the help of the National Cyber Security Centre.
We're aware of a possible cyber incident affecting parliament & are actively cooperating with @ncsc who're leading the operational response pic.twitter.com/YOvPwQvn37
— NationalCrimeAgency (@NCA_UK) June 24, 2017
British Labour Party leader Jeremy Corbyn told a crowd at the Glastonbury festival: “I think (this) indicates just how vulnerable we are to cyber-attacks and our cyber-security.
“We need to be investing in cyber protection – it is a huge issue. We all rely on computers, we all rely on emails, we all rely on digital records. You wouldn’t leave your building without importing documents under lock and key. A computer is just the same.”
Commenting on the incident, Rob Homes, vice-president of products at security company Proofpoint in EMEA, said: “The news that the UK parliament has suffered a sustained cyber attack aimed at all user accounts is both alarming and unsurprising.
"This seems to be a relatively unsubtle attack aimed at all parliamentary user accounts to find weak passwords. As organisations (including Parliament with Office365) have moved to cloud email and collaboration platforms, attackers have adapted their tradecraft away from targeting networks to targeting people and their credentials.
"As we’ve seen in prior attacks on governments and politicians, few things are as valuable as a compromised email account. The recommendations remain the same: regularly change your login credentials, use multi-factor authentication that conforms to best practice, and invest in defensive technologies that can protect against attacks targeted at cloud environments."