Amnesty International is urging senior judges not to sit on the public inquiry into the murder of Belfast solicitor Pat Finucane.
Mr Finucane, 39, was shot dead in front of his family
The human rights group said the "sham" inquiry, set up under the Inquiries Act 2005, had been "railroaded" through parliament and would lack independence.
Mr Finucane, 39, was killed by loyalist paramilitaries at his home in 1989.
In a statement last December, the Northern Ireland Office said nothing would be withheld from the inquiry.
However, Amnesty International's UK director, Kate Allen, said crucial evidence could be omitted from any final report at the government's discretion.
"Any judge presiding over an inquiry into the Finucane murder, under the Inquiries Act 2005, would be presiding over a sham. We urge judges not to sit on any such inquiry," she said.
Judge Cory has criticised government inquiry plans
"By rushing through this act, the government has placed itself beyond public scrutiny and dealt a massive blow to any hopes of transparency in government.
"Under the Inquiries Act 2005, there will be no more independent, public inquiries like those into the Ladbroke Grove train crash, the murder of Stephen Lawrence or the tragedy at Hillsborough.
"The government will be able to control what the public finds out, and what it doesn't."
The comments have been backed by the SDLP's Alban Maginness.
He said: "Amnesty International is quite right. No self-respecting judge should sit on an inquiry that would not be independent and that British government ministers could muzzle at any time.
"Such an inquiry would be a sham, the sort of thing one would expect from totalitarian regimes, not from a democratic government."
In March, before the act was approved by parliament, it was criticised by the judge who investigated allegations of security force collusion in Mr Finucane's murder.
Retired Canadian judge Peter Cory said the proposed new legislation "would make a meaningful inquiry impossible".
He said the bill would set up "impossible terms for any international judge asked to chair the inquiry".
Mr Finucane's son John also said the government should reconsider changes to the law.
The Finucane family have concerns about the independence and the powers of the inquiry and said it did not comply with Judge Cory's recommendations.
Mr Finucane's killing was one of the most controversial of the 30 years of the Troubles in Northern Ireland, mainly because of the allegations of collusion between the Ulster Defence Association and members of the security forces.
In 2001, Judge Cory was appointed by the British and Irish governments to examine allegations of collusion surrounding the Finucane and other controversial killings.
Judge Cory recommended a public inquiry into Mr Finucane's death.
Last December, the NIO said the government wanted the inquiry to be able to get at the full facts of what happened.
However, because of national security, it said a large proportion of evidence would "have to be considered in private".
Loyalist Ken Barrett, 41, was sentenced in September to life for Mr Finucane's murder, after admitting his part in the killing.