Chakrabarti, a London-based prominent Indian-origin human rights activist, said any such clampdown would breach the Human Rights Act and be open to legal challenge.
The Leveson's report last week recommended an independent self-regulatory body for the British media industry, backed up by legislation. Currently, the British press is self-regulated through the Press Complaints Commission (PCC).
British Prime Minister David Cameron had set up the inquiry last year after it emerged that journalists at the now-defunct News of the World tabloid of Rupert Murdoch had hacked the phone of Milly Dowler, a 13-year-old murdered schoolgirl, as well as targeted dozens of crime victims, celebrities and politicians.
Chakrabarti, 43, one of six assessors who worked on the Leveson Inquiry, said: "We were chosen as advisers because of our areas of expertise.
"Mine is human rights law and civil liberties. In a democracy, regulation of the press and imposing standards on it must be voluntary," she told the 'Daily Mail'.
"A compulsory statute to regulate media ethics in the way the report suggests would violate the act, and I cannot support it," Chakrabarti, director of the civil rights group Liberty, said.
"It would mean the press was being coerced in being held to higher standards than anyone else, and this would be unlawful," she said.
At the heart of Chakrabarti's objections to the Leveson report is that any new law that made the government quango Ofcom the "backstop regulator" with sweeping powers to punish newspapers would violate Article 10 of the European Convention On Human Rights which guarantees free speech and is enshrined in Britain's Human Rights Act, too.
Her intervention is hugely significant because as one of only six 'assessors' who helped guide the Levenson inquiry and its conclusions, her position threatens the viability of key parts of the report, the paper Daily Mail' said.
Chakrabarti said that if the recommendation were made law, any publication which became subject to compulsory regulation would be able to challenge it in the courts, and seek a legal declaration that the 'Leveson law' flouted human rights.
If the law were not repealed, the publication could seek to have it overturned at the European Court of Human Rights in Strasbourg
Article 10 states: "Everyone has the right to freedom of expression. This right shall include freedom to hold opinions and to receive and impart information and ideas without interference by public authority and regardless of frontiers."
Chakrabarti also rebutted Leveson's claim that a new statute to establish a press regulator is "essential". She said the new regulator does not need "statutory underpinning", and should not rely on approval from Ofcom or any other quango.
The important thing, she said, was for the UK newspaper industry to set up as quickly as possible a new, independent regulator along the lines Leveson recommends, with real powers to punish publications. The debate over whether any change in law was then required should wait until its structure was clear, she said.
But neither Ofcom nor any other state quango should validate it, she said.
"I don't think quangos such as Ofcom are independent enough of the state to be judging press regulation. I don't think it's necessary for the Leveson scheme to work.
"Quangos are an emanation of the state, and their members are appointed by Ministers," she said.
If a quango were to be involved, she added, Ofcom was an especially unfortunate choice, because its existing role in governing broadcasters means it is a "regulator of content".
For example, Ofcom has powers to decide whether programmes breach broadcasting rules on political bias, which have never applied to the press, and to punish those who transgress.
Leveson's recommendation that Ofcom should be the body empowered by law to decide whether the regulator was doing its job was "something I can never accept", she said.
"Journalism must never become a state-licensed profession". Some, she noted, had argued that the state does license drivers, implying that licensing for the media would be no more onerous.
"But driving a car isn't a fundamental human right," she said.
London: Shami Chakrabarti, a top adviser to Lord Justice Leveson says his key proposal for compulsory press regulation in UK in the aftermath of the phone-hacking scandal would be illegal as it would breach the Human Rights Act.
First Published: Sunday, December 02, 2012, 15:40