Alastair Campbell will give evidence at the press standards inquiry - although his evidence has already been leaked to a blogger.
The former Number 10 communications director will give oral evidence at the Leveson Inquiry.
A draft of his written evidence was posted online on Sunday by Paul Staines, who blogs under the name Guido Fawkes. The move prompted Lord Justice Leveson to make an order banning advance publication of documents submitted to the inquiry.
He also summoned Mr Staines, whose Order-Order blog includes political gossip, comment, and genuine news scoops, to give evidence to the inquiry about how he obtained Mr Campbell's statement. The blogger, who claimed to have obtained it "by legal means", is expected to appear before the inquiry on Thursday.
On Wednesday the inquiry will also hear from former Merseyside Police Inspector Alec Owens, who led Operation Motorman - the 2003 Information Commissioner's Officer (ICO) investigation into the media's use of inquiry agents in 2003.
After the operation, inquiry agent Steve Whittamore was convicted in 2005 of illegally accessing data and passing it to journalists. Mr Owens has since criticised the fact journalists named in the transactions with Whittamore were not questioned.
On Tuesday the inquiry heard claims that News of the World (NotW) journalists hacked phones for their former editors Rebekah Brooks and Andy Coulson.
Paul McMullan, who worked for the now-defunct Sunday tabloid for seven years, said editors were aware their staff were illegally accessing voicemail messages but threw them "to the wolves" by denying all knowledge of it.
He described former editor Mrs Brooks as "the criminal-in-chief", alleging she "moulded" David Cameron before he became Prime Minister.
Ms Brooks and Mr Coulson have denied all knowledge of phone hacking.