Media playback is unsupported on your device Media caption Shami Chakrabarti tells the BBC's Andrew Marr that she does support Leveson's report

A key adviser to Lord Justice Leveson claims his last-resort option for compulsory press regulation if they did not join voluntarily could be illegal.

The judge's report recommended an independent self-regulatory body for the industry backed up by legislation.

But, if newspapers did not sign up to that, he said politicians might need to consider compulsory regulation.

Liberty's Shami Chakrabarti said she could not support this as it would breach the Human Rights Act.

She said it could have "serious unintended consequences".

'Carrots and sticks'

Ms Chakrabarti, one of six assessors who worked on the Leveson Inquiry, told the BBC's Andrew Marr programme she supported the "carrots and sticks" in the report, that meant newspapers who signed up to a new watchdog would be subject to lower penalties.

"The bombshell, or the difference, is what do you do if people don't join the club or don't set up a club. And Leveson doesn't want compulsory regulation of the press, but he says if they don't play ball, politicians may have to consider it. That is where I get off the bus," she said.

She told the Mail on Sunday that, in a democracy, regulation of the press and imposing standards on it must be voluntary.

"A compulsory statute to regulate media ethics in the way the report suggests would violate the act, and I cannot support it."

Proposed new press law Would: Create a process to "validate" the independence and effectiveness of the new self-regulation body

Validate a new process of independent arbitration for complainants - which would benefit both the public and publishers by providing speedy resolutions

Place a duty on government to protect the freedom of press Would not: Establish a body to regulate the press directly

Give any Parliament or government rights to interfere with what newspapers publish How new regulator might work

However deputy Labour leader Harriet Harman said she did not think Ms Chakrabarti was right.

"What Leveson proposes is actually quite similar to the Irish system, and the Irish are passionate supporters of the European Convention on Human Rights, and their system, which is backed by law, has not fallen foul from the European Convention, so she was wrong about that," she told the BBC's Sunday Politics show.

Mrs Harman added that Labour was "absolutely not proposing that public authority should interfere with the freedom of the press".

Labour leader Ed Miliband has said Prime Minister David Cameron must back Lord Justice Leveson's proposal by Christmas or Labour will pull out of cross-party talks.

Mr Miliband told the Observer the prime minister had "one last chance to show leadership" or Labour would begin rallying for a Commons vote in January, which could see him defeated.

Labour is planning to draw up its own draft bill, which it said it would put to the Commons if the government failed to produce a workable bill.

Online petition

The prime minister has indicated he has "serious concerns and misgivings" about legislation to regulate the press.

Chancellor George Osborne told the BBC he did not think Labour should be setting a deadline and it was important that the parties continued talking to find a consensus.

Cross-party talks on the 2,000 page report are set to resume on Monday before MPs debate its contents.

Campaign group Hacked Off, which represents people who have been victims of press intrusion, is running a petition for Lord Justice Leveson's recommendations to be implemented in full. It now has about 100,000 signatures.

Its director, Brian Cathcart, said: "We hope that the prime minister, who last Thursday appeared to reject a key part of the recommendations, is listening to the voice of the public, just as he promised he would in his evidence under oath at the Leveson Inquiry."

David Cameron is due to attend a meeting on Tuesday with Culture Secretary Maria Miller and newspaper editors.

Meanwhile, Alex Salmond has invited opposition party leaders to discuss how Lord Justice Leveson's report into press standards could be implemented in Scotland.

The first minister will hold talks with his fellow MSPs on Thursday in an attempt to find "cross-party consensus" on a new system of press regulation.