The "sustained" hack began on Friday night, prompting officials to disable remote access to the emails of MPs, peers and their staff as a safeguard, BBC reported on Saturday.
The parliamentary authorities said hackers had mounted a "determined attack" on all user accounts "in an attempt to identify weak passwords".
A spokeswoman said they were investigating and liaising with the National Cyber Security Centre.
She said: "We have discovered unauthorized attempts to access accounts of parliamentary networks users".
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".
IT services on the parliamentary estate are working normally and a message sent to MPs urges them to remain "extra vigilant".
But a number of MPs have confirmed to the BBC they are not able to access their parliamentary email accounts outside of the Westminster estate.
It comes just over a month after 48 of England's NHS trusts were hit by a cyber-attack
The latest attack was publicly revealed by Liberal Democrat peer Lord Rennard on Twitter who had also asked his followers to send any "urgent messages" to him by text.
Henry Smith, Tory MP for Crawley, tweeted: "Sorry no parliamentary email access today - we're under cyber attack from Kim Jong Un, (Vladimir) Putin or a kid in his mom's basement or something".
The government's National Security Strategy said in 2015 that the threat from cyber-attacks from both organized crime and foreign intelligence agencies was one of the "most significant risks to UK interests".
The National Cyber Security Centre, which is part of intelligence agency GCHQ, started its operations in October last year.
The National Crime Agency said it was working with the NCSC but the center was "leading the operational response".
News Source: news18Last modified on Thursday, 28 September 2017