Former Conservative Prime Minister Sir John Major, Labour Leader Ed Miliband and his deputy Harriet Harman are due before the Leveson Inquiry later.
The third module of the media ethics inquiry is focusing on the relationship between the press and politicians.
On Monday, Lord Justice Leveson heard from former Prime Minister Gordon Brown and the Chancellor George Osborne.
Prime Minister David Cameron will give evidence to the inquiry at London's Royal Courts of Justice on Thursday.
Mr Miliband has previously said Mr Cameron's government "bent over backwards" to help Rupert Murdoch's media empire but that Labour had also been "too close to the powerful interests" when it was in power.
No 'vast conspiracy'Mr Osborne told the inquiry on Monday that it would be "complete nonsense" to believe there was a "vast conspiracy" to hand control of BSkyB to Rupert Murdoch.
He said News Corp's £8bn bid for the broadcaster had been a "political inconvenience".
Mr Osborne also defended ex-News of the World editor Andy Coulson's appointment as Tory Party director of communications saying he was appointed because of his abilities, not his contacts.
In his evidence on Monday, Mr Brown said NHS chiefs had apologised to him after finding it was "highly likely" details of his son's cystic fibrosis were disclosed by a staff member.
The Sun ran a story in 2006 about Fraser Brown's medical condition, but denied accessing his medical records. Mr Brown insisted that he and his wife did not grant the Sun permission for the story.
Mr Brown also said a phone call in which Rupert Murdoch told the inquiry he had threatened to wage war on News Corporation had never happened.
Deputy Prime Minister Nick Clegg and Scotland's First Minister Alex Salmond will appear before the inquiry on Wednesday.
