Labour leader Ed Miliband and former prime minister Sir John Major will give evidence to the Leveson Inquiry into press standards today.
Mr Miliband became a strong critic of Rupert Murdoch's News International after the scale of the News of the World phone-hacking allegations emerged last summer.
But he has had a string of contacts with News International executives since becoming Labour leader.
He is also likely to be questioned about relations between Gordon Brown's government - in which he was a Cabinet minister - and the media.
Sir John received an increasingly torrid press during his time in Downing Street and is said to have taken newspaper criticism to heart.
The Sun famously switched its support from the Tories to Labour for the 1997 general election, when his premiership was brought to an end.
Labour deputy leader Harriet Harman will also be questioned at the inquiry today.
Yesterday former prime minister Gordon Brown, appearing at the inquiry, denied claims that he had "declared war" on Rupert Murdoch in a telephone call to the media mogul or behaved aggressively towards News International chief executive Rebekah Brooks.
He said the conversation where he was "alleged to have acted in an unbalanced way" as well as threatening Mr Murdoch "never took place".
"I'm shocked and surprised that it should be suggested even when there is no evidence of such a conversation," he said.
He also criticised the way The Sun published a story about his son Fraser's cystic fibrosis, which he complained about last July.
The ex-Labour leader called for measures to stop the "conflation of fact and opinion" in the press.