Parliamentary authority have confirmed MPs and Peers have been left unable to remotely access their emails as a result of a cyber security incident.
A House of Commons spokeswoman said: "The Houses of Parliament have discovered unauthorised attempts to access parliamentary user accounts.
"We are continuing to investigate this incident and take further measures to secure the computer network, liaising with the National Cyber Security Centre.
"We have systems in place to protect member and staff accounts and are taking the necessary steps to protect our systems."
The spokeswoman said that the lack of remote access for users was not part of the attack, but due to protection measures dealing with the incident.
She said: "Parliament has disabled remote access to protect the network."
Liberal Democrat peer Baron Rennard tweeted: "Cyber security attack on Westminster, Parliamentary emails may not work remotely."
The attack follows reports that passwords for MPs and officials were being sold online by hackers.
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 National Crime Agency said it was working with the NCSC on the issue.
It tweeted: "We're aware of a possible cyber incident affecting Parliament & are actively cooperating with @ncsc who're leading the operational response."
International Trade Secretary Liam Fox told ITV News that the attack was a "warning to everyone we need more security and better passwords. You wouldn't leave your door open at night."
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."
Parliamentary authorities are believed to have been alerted to the attack on Friday.
An email sent by parliamentary authorities to those impacted by the incident described the attack as "determined", according to The Guardian.
It said the email stated: "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."