Labour leader Ed Miliband has insisted that Lord Justice Leveson's proposals for the reform of press regulation must be swiftly implemented by Parliament as long as they are "reasonable and proportionate".
He warned that if the Government rejected the Leveson report, victims of newspaper harassment and intrusion would see it as a "breach of the promise" made to them by politicians in the wake of the phone hacking scandal.
His comments come ahead of the report's publication on Thursday and as more than 40 Conservative MPs are preparing to issue an appeal to Prime Minister David Cameron to avoid statutory regulation.
The MPs are set to publish a letter calling for a tougher system of self-regulation - something that Mr Miliband, Liberal Democrats and the Hacked Off campaign group feel is inadequate.
On Sunday, Foreign Secretary William Hague became the latest Cabinet minister to hint his reluctance to support statutory regulation, urging that the Government should "err on the side of freedom".
The Foreign Secretary said he was a "big supporter of press freedom" although he stressed he wanted to read the Leveson report before giving his verdict on it. "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 BBC One'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."
Mayor of London Boris Johnson warned: "If you go around sterilising, pasteurising and homogenising the media you will have a bad effect on our democracy."
Downing Street said Mr Cameron was keeping an open mind and would make no decisions before he has seen the report arising from the Leveson Inquiry which he set up in the wake of last year's phone-hacking scandal.
The Conservative Party is divided on the issue of press regulation - 42 Tory MPs led by Mr Cameron's former press secretary George Eustice signed a letter this month calling for the door to be kept open on some kind of statutory regulation.
Writing in The Guardian, Mr Miliband said that proposals by the newspaper industry to retain self-regulation would "leave the press, despite all the scandals of the last few years, as the only major source of power in Britain trusted to regulate itself". "The Prime Minister was absolutely right when he said that if Leveson's recommendations were 'bonkers', we could not be expected to go along with them," he wrote. "But it is equally right to state that if they are reasonable and proportionate, we should seek to implement them. From our meeting with the victims last week I know that rejection of the report will be seen as a clear breach of the promise we made to them. If Parliament chooses a different course from that recommended there must be clearly demonstrated and very good reasons for doing so."