Iran carried out a cyber attack on parliament this summer, according to reports.
MPs including Theresa May and other cabinet officials were hit by the June attack, which affected around 9,000 email accounts. In total, around 90 were compromised.
The Times first reported the "sustained and determined" attack was traced to Tehran, citing a secret intelligence assessment.
Blame over the cyber attack, which identified weak email passwords, was initially pointed at Russia. Now it is believed to be Iran's "first significant act of cyberwarfare" on Britain.
The hack came after dozens of NHS hospitals were crippled by a massive ransomware attack over the summer.
This revelation comes just as Britain is defending its commitment to preserving the Iran nuclear deal.
The Prime Minister issued a joint statement in support of the pact along with the German Chancellor and French President after Donald Trump refused to certify the 2015 deal last night.