A legal challenge over the recognition of Impress as a new regulatory body for the press has reached the High Court.
The Press Recognition Panel (PRP), set up under a Royal Charter after the Leveson inquiry into the media, gave its approval to Impress, which is backed by the former motor sport mogul Max Mosley, in October.
Impress has few members, and most national newspapers have instead signed up to the Independent Press Standards Organisation (Ipso), a voluntary independent body not backed by the Government.
The press industry fears the go-ahead for Impress could trigger legislation that could force local or national newspapers to pay the costs of libel or privacy actions against them, even if they win their cases, potentially forcing them out of business.
The News Media Association, which represents publishers, is bringing the case in London on Thursday before Lady Justice Rafferty and Mr Justice Popplewell.
Hailing the PRP's decision, Dr Evan Harris, from Hacked Off, which campaigns for an accountable press, has said: "This makes Impress the only regulator which the public, readers and victims of press abuse can trust to regulate newspapers and safeguard freedom of the press, while offering redress when they get things wrong."
But newspaper organisations fear the move could see them "blackmailed" into signing up to a state-backed system of regulation, as those that do so would be exempt from such laws.
The industry has warned that press freedom is in peril, and leading newspapers have called on Prime Minister Theresa May to refuse to implement the legislation, under section 40 of the Crime and Courts Act 2013 (CCA).