The DUP's Sir Jeffrey Donaldson has blasted a cyber attack which has locked his party's MPs and staff out of their parliamentary emails since Friday as "a very serious attack".
Hackers targeted Parliament's email system in an attempt to access the accounts of hundreds of MPs, Lords, aides and staff.
The attack is believed to have taken place at some point last week but the full effect became known on Friday when MPs were informed of what happened.
Around 90 accounts are believed to have been compromised. It is not known what group was behind the attack, although The Guardian has reported claims that suspicion had fallen on Russia and North Korea.
Security services shut down access for anyone not in Westminster as part of efforts to secure the network after the allegedly state-sponsored attack.
Lagan Valley MP Sir Jeffrey Donaldson said while his party had not yet been told whether any of their members were hacked, they were unable to access their parliamentary emails for 24 hours over the weekend.
A DUP spokesman told the Belfast Telegraph that many of its MPs and staff were still unable to access their emails.
"This was a very serious attack on Parliament and on our democracy and as a result, MPs have lost access to their emails and to the parliamentary intranet," Mr Donaldson said.
The MP has also called for an urgent review of Westminster's IT system.
"This will mean some constituents emails not being seen for a period of time," he said.
"Clearly, the parliamentary authorities need to urgently review the security of the parliamentary IT system so as to prevent further cyber attacks on our democratic institutions in the future."
A parliamentary spokesperson said yesterday: "Parliament's first priority has been to protect the parliamentary network and systems from the sustained and determined cyber attack to ensure that the business of the Houses can continue.
"This has been achieved and both houses will meet as planned today.
"Investigations are ongoing, but it has become clear that significantly fewer than 1% of the 9,000 accounts on the parliamentary network have been compromised as a result of the use of weak passwords that did not conform to guidance issued by the parliamentary digital service.
"As they are identified, the individuals whose accounts have been compromised have been contacted and investigations to determine whether any data has been lost are under way."
Sinn Fein's seven MPs who do not take their seats but do use their parliamentary emails did not respond when asked whether they had been affected by the hack.
North Down MP Lady Sylvia Hermon said her email account had not been affected.