MPs have been warned that they face blackmail threats after a “brute force attack” on their email accounts, which authorities believe was state sponsored.
A senior security source told the Sunday Times it was “inevitable” some information would have been stolen in the attack.
Emergency changes were made to MPs’ IT system on Friday after a “cyber security incident”.
A high-importance email was sent out by Parliamentary officials warning MPs to ‘remain extra vigilant’ and look out for ‘suspicious emails or activity’.
The email, sent just after lunchtime, said: “We are currently investigating a cyber security incident.
“We are going to be making some emergency changes to the IT network which may cause some disruption.”
The email was sent hours after reports that British politicians’ passwords had been traded by Russian hackers.
The Times claimed 1,000 MPs and parliamentary staff, 7,000 police employees and more than 1,000 Foreign Office staff were in the troves sold or swapped on Russian-speaking hacking sites.
Cabinet ministers including Education Secretary Justine Greening and Business Secretary Greg Clark were said to be among the haul.
However, a House of Commons spokesman would not confirm whether or not the probe was related to The Times’ report.
Officials also declined to say how many email accounts were affected, when the incident occurred or what it was.
The spokesman said: “The Houses of Parliament are currently investigating a cyber security incident.
“Like all responsible organisations we have to respond to constantly evolving threats. We have systems in place to protect member and staff accounts and to prevent unauthorised access to information. We are working to investigate the incident and will be taking the necessary steps to protect our systems.”