Liberal Democrat lawmaker Chris Rennard tweeted that there was a "cyber security attack on Westminster; Parliamentary e-mails may not work remotely."
A House of Commons spokesman confirmed that "the Houses of Parliament have discovered unauthorised attempts to access parliamentary user accounts.
"We have systems in place to protect member and staff accounts and are taking the necessary steps to protect our systems," it added.
The threat follows reports in British media, including the Times, that hackers were selling passwords for MPs online.
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