Hugh Grant says he will match the £1 million fines imposed on newspapers under David Cameron's proposed press controls with his own money, should they be passed in the House of Commons on Monday 18 March.
Speaking on the BBC's Andrew Marr Show, the actor poured scorn on Conservative plans for a press watchdog set up by Royal Charter in the wake of the Leveson report into press standards, and said he had personally spoken to members of the shadow cabinet to urge them to vote against the proposals.
"They say we are going to have £1m fines. Well, there won't be £1m fines under their scheme. If their scheme is set up and there's a £1m fine, I will match it," said Grant. "They've designed an investigative scheme so labyrinthine and complex that it would never come to the point where a £1m fine was actually levied."
Cross-party talks on how to implement Leveson's proposals broke down last week, with the prime minister opting to put his own plans for a regulator backed by Royal Charter before parliament for a vote.
Labour and the Lib Dems will table rival plans that envisage writing the role of the new watchdog into statute.
"It's become a huge thing, but the sands are shifting all the time," Grant told the BBC. "The basic choice for MPs is for a Royal Charter, designed by the prime minister in conjunction with the press industry - government as dictated to by the press - as a means of the prime minister saving his scalp."
Grant, who is a leading member of the Hacked Off campaign set up in response to allegations of phone hacking at the now-defunct News of the World, accused the prime minister of reneging on his promise to protect victims of press intrusion from the worst excesses of the industry.
"All three party leaders did say at various junctures, including under oath at Leveson, and to the victims themselves - people like the Dowlers and McCanns, et cetera - what matters most is that people like them should be protected. The prime minister has completely gone back on that promise."
He accused Cameron of conniving with the interests of the press to set up a system that would largely maintain the status quo.
"The old regulator, the Press Complaints Commission, was the press marking its own homework, so you got terrible, egregious abuses of innocent people, which could happen with no penalty at all. The new charter is cunningly designed to go right back to the PCC.
"There are either glaring omissions in terms of Leveson's recommendations, where the press can write their own code, or little tiny moth holes designed by press lawyers that can later be exploited."
He confirmed that he had personally lobbied MPs from all three parties to reject Cameron's proposals. "I called some members of the shadow cabinet, I certainly did," said Grant. "I think it was on Tuesday when we heard a rumour - we still don't know if it's true or false - that there might be an early compromise, and we felt it was much too early."
He predicted that Cameron would lose Monday's vote if Tory backbenchers stuck to their guns and voted with their conscience. "At the moment it's close, but it looks like if the minority parties hold firm, the Tory rebels will vote with their conscience. They feel quite rightly that Cameron's pledge to do right by the victims should be met."