British government sources say a cyber attack on the Westminster parliamentary computer system appears to have been contained.
Officials first noticed unusual activity on Friday morning and it has been described as a sustained and determined operation.
Hackers were probing email accounts to see if any used weak passwords and if the accounts could be accessed.
It is unclear if any accounts were breached, or who was behind the attacks, but the concern is that sensitive information could have been stolen.
The National Cyber Security Centre 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 told the Press Association 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".
International Trade Secretary Liam Fox said the incident was no surprise after the recent major hacking attack against the NHS.
Mr 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 yesterday, 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."