Ed Miliband has claimed the Leveson inquiry represents a "once in a generation" opportunity to change the press - as Boris Johnson warned against "sterilising, pasteurising and homogenising" Britain's media.
The comments from the Labour leader and Conservative Mayor of London come ahead of the Lord Justice Leveson's report, which is due to be published on Thursday.
Miliband, writing in the Guardian, said he would implement Leveson's recommendations, providing they are not "bonkers."
"This is a once in a generation opportunity for change. We owe it to the victims – and to the country – to seize it," he wrote.
The Labour leader, who supports "independent regulation of the press, made possible by statute" said parliament should debate Leveson's recommendations.
Miliband said if the Government rejected the Leveson report, victims of newspaper harassment and intrusion such as the Dowlers family would see it as a "breach of the promise" made to them by politicians in the wake of the phone hacking scandal.
But Boris Johnson, speaking to Radio 5 Live, said he was a "bit nervous."
"I am a bit nervous we are heading in the opposite direction to many other countries in the world which are liberating their press and allowing free speech, and I think statutory regulation is not something that I would support."
It came after Foreign Secretary William Hague said he was a "big supporter of press freedom" but said he wanted to read Leveson's report before passing judgement.
He claimed "none of us [the cabinet]" had yet seen Lord Justice Leveson's recommendations.
"Although I'm a big supporter of the freedom of the press, I'm also a big supporter of actually reading something before you pronounce on it," he told BBC1's Andrew Marr Show.
"We will have to do that, but in my case, from the philosophical viewpoint that you have to err on the side of freedom."