Blogger Guido Fawkes has been called to the public inquiry on press ethics to explain how he published Alastair Campbell's witness statement before it was released.
The Leveson Inquiry has been hearing from Chris Jefferies, the landlord wrongly held on suspicion of murdering of Joanna Yates, who said he was vilified by the media .
But during the weekend, a copy of Mr Campbell's written submission to the panel was published online, days before Tony Blair's former director of communications is due to give evidence.
Paul Staines, who blogs as Guido Fawkes at Order-Order.com , said he obtained the document "by legal means".
But the chair of the inquiry, Lord Justice Leveson, has said he expects Mr Staines to attend this week and explain his actions.
He refused to identify Mr Staines by name, adding: "I am concerned to deprive the particular website of that oxygen."
Lord Justice Leveson said he was worried about the security of the computer system holding confidential witness statements.
He has also asked lawyers representing "core participants" to check their own arrangements for ensuring documents are kept secret.
"I am obviously concerned about the security of the information that is available and to maintain the integrity of the inquiry as we move forward," he said.
"As a result I am intending to inquire to such extent as I can into the circumstances in which this statement came to be available for publication."
The chair also passed an order banning any inquiry documents from being leaked.
A statement on the inquiry's website said Mr Campbell's final written evidence differed slightly to the version that appeared online.
Mr Campbell, the former prime minister's spin doctor for the majority of his time in Downing Street, wrote on Twitter: "Genuinely shocked someone has seen fit to leak my statement to Leveson. Less surprised that Guido Fawkes headline misrepresents it."
The Leveson Inquiry was set up at the height of the phone-hacking scandal. It is examining the ethics of the media and will hear from a range of alleged victims of intrusion.