According to Sky News, the attacks took place on 23 December, with more than 10,000 pieces of data records stolen.
The data stolen included email addresses, postal addresses, company positions and phone numbers, including the mobile number of Post Office chief executive Paula Vennells and at least ten peers and MPs.
Other private companies and banks were also affected, cyber security experts said, blaming a Iranian Revolutionary Guard group for the incident and a previous attack hitting the parliamentary network in 2017.
British security services have not confirmed Iran’s involvement, but the National Cyber Security Centre said it was “aware of a cyber incident affecting some UK organisations in late 2018”.
“Political disruption provides a fertile ground for cyber-attacks against government, non-government and international organisations, meaning it’s hardly surprising malicious actors in Iran have mounted an attack against the UK,” Darren Anstee, chief technical officer at IT specialist firm Netscout, said.
“Attacks by Iran can be effective, as groups in the country are known to be employing new techniques, as well as combining custom-made tools with commodity crimeware to extend their reach and impact.
“As a result, it is critical that governments and organisations make themselves aware of these new methods to disrupt and interfere with domestic and international affairs. It is also essential that governments and businesses collaborate closely to neutralise threats and prevent attacks on national institutions.”
The news comes as social networks continue to fight interference from Iran and other countries seeking to manipulate political events elsewhere.
In February, Facebook said it had removed 783 pages, groups and accounts “engaging in co-ordinated inauthentic behaviour tied to Iran” using re-purposed news stories from Iranian state media, about topics such as Israeli-Palestinian relations and the conflicts in Syria and Yemen.
The bad actors misrepresented themselves as originating from other countries, but were traced back to Iran.