Hackers hit the Westminster network with a “sustained and determined” bid to break into the email accounts of MPs, peers and their staff.
Users of the Westminster system have been unable to access their accounts remotely due to defensive measures put in place by parliamentary authorities to deal with the hack.
The National Cyber Security Centre (NCSC) and National Crime Agency are probing the incident, which comes after reports that passwords for Cabinet ministers and MPs were being sold online by hackers.
Tory MP Andrew Bridgen said such an attack “absolutely” could leave some people open to blackmail.
He said: “Constituents want to know the information they send to us is completely secure.”
An email sent by parliamentary authorities to those impacted by the attack, which is believed to have begun on Friday, said that after “unusual activity” was spotted, “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”.
A parliamentary spokesman said: “We have discovered unauthorised attempts to access accounts of parliamentary networks users and are investigating this ongoing incident, working closely with the National Cyber Security Centre.
“Parliament has robust measures in place to protect all of our accounts and systems, and we are taking the necessary steps to protect and secure our network.
“As a precaution we have temporarily restricted remote access to the network. As a result, some Members of Parliament and staff cannot access their email accounts outside of Westminster.
“IT services on the parliamentary estate are working normally. We will continue to keep members of both Houses of Parliament and the public updated as the situation develops.”
International Trade Secretary Liam Fox said the incident was no surprise after the recent major hacking attack against the NHS.
Dr 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.
“And it’s a warning to everybody, whether they are in Parliament or elsewhere, that they need to do everything possible to maintain their own cyber security, including having complex and therefore safer code words.”
Speaking at Glastonbury Festival, Labour leader Jeremy Corbyn said: “I think it 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.”
The National Crime Agency tweeted: “We’re aware of a possible cyber incident affecting parliament & are actively cooperating with @ncsc who’re leading the operational response.”
Additional reporting by Press Association.