The political consensus over a new system of press regulation is running into difficulty with some of the country's key newspaper groups, including the publisher of the Daily Mail and the Sun, refusing to endorse it.

One senior source said the groups were taking "high-level legal advice" before deciding if they could join the new watchdog or not.

In a statement signed by Associated Newspapers, News International, the Telegraph Media group and the Express publishers, Northern & Shell, they said the deal included "several deeply contentious issues which have not yet been resolved with the industry".

It is understood the group has issues with the statute that would be laid down to guarantee no change to the royal charter, along with concerns over the proposals to give the regulator the power to force apologies.

A refusal to back the deal would set the newspapers at odds not only with the three main political parties, but with many of the victims of hacking and the anti-hacking pressure group Hacked Off.

The newspapers strongly oppose the proposal that, for the first time, people from outside the industry would be involved in drawing up the newspaper code of practice, a code that has been widely praised and has been adapted by regulators in other countries.

"This is a political deal between the three parties and Hacked Off. It is not a deal with the newspapers," said a senior executive in one newspaper group.

Trevor Kavanagh, the associate editor of the Sun, said it was worrying "when three political parties get together and their final verdict is welcomed so enthusiastically by Hacked Off which is definitely seeking to shackle and gag the free press. We simply do not want politicians to have control whatsoever in what goes in or doesn't go into newspapers."

Associated Newspapers, News International and the Telegraph Media Group had been exploring the possibility of boycotting the government-sanctioned regulator and setting up their own if they believed it could threaten freedom of the press. One source said this was still "very much a live discussion".

"Nobody is threatening it, or saying we will do it, but we won't be making a decision before we have had high-level legal advice," said the insider, who claimed that the existence of a "no change" statute to guarantee the royal charter could not be amended by the privy council still opened the door to political interference.

"The whole concept of a royal charter as sold to the industry has been undermined by the idea that it needs a parliamentary decision to change it. That's the anxiety," the source said.

A second senior executive said the industry had already been advised that the proposal that the regulatory body could force newspapers into making apologies would be illegal as it would be contrary to article 10 of the European convention of human rights, which protects freedom of speech.

"No courts impose apologies on newspapers. Statements and apologies are always agreed by both sides. We have got legal opinion this could be against article 10 because it forces people to write something they don't agree to," the source said.

The Financial Times said the onus was on the industry to make the regulation work but its leader comment expressed reservations about the "no change statute" for the royal charter. "This is either artful or downright disingenuous," it said adding that "legislation, even in a simple clause, sets a worrying precedent".

Jonathan Dimbleby, who chairs Index on Censorship, condemned the royal charter statute, saying the board had "the gravest anxiety at the residual political powers the now-expected outcome and system will give to politicians".