The Irish parliament has backed an all-party motion calling for a full public inquiry into the 1989 murder of Belfast solicitor Pat Finucane.
Mr Finucane, 39, was shot dead in front of his family
The all-party motion called for the British government to hold "a full, independent, public judicial inquiry".
The NIO said the motion was "fundamentally flawed and misleading".
Mr Finucane's murder by the UDA was one of the most controversial of the Troubles due to allegations of security force collusion.
His family have said they do not think an inquiry held under the Inquiries Act would be able to get to the truth.
Moving the motion on Wednesday, Irish Foreign Affairs Minister Dermot Ahern said the Irish government had consistently raised the issue over several years with the British government, the European parliament, the Council of Europe and the United Nations.
"The position of the Irish government remains firm and emphatic. We ask the British government to establish a full, independent, public, judicial inquiry into the murder and nothing less," he added.
Labour Party TD Michael D Higgins accused Downing Street of taking a "strategic decision" to withhold the truth on the Finucane death.
"One can only conclude that the British government want to indulge in a major cover-up in order to prevent the true nature of the collusion between the Royal Ulster Constabulary and the loyalist paramilitaries who murdered Pat Finucane coming into the public domain."
Mr Finucane's widow, Geraldine, and sons Michael and Dermot were in the Dail visitors' gallery to hear the debate.
Opposition leader Enda Kenny, who originally suggested the proposal to Mrs Finucane, said the world had lost a human rights defender as well as a loving husband and adoring father in the most savage of circumstances.
The Fine Gael leader said: "In this time of new and fragile peace, it behoves the British government to confront, unequivocally, what is a major disquiet for people north and south.
"The inquiry into the murder of Pat Finucane must be forensic, independent and public. In terms of both justice and human decency, it is long overdue. It is needed now."
Retired Canadian judge Peter Cory recommended separate inquiries into Mr Finucane's murder, and three other controversial killings in Northern Ireland.
These were the killings of solicitor Rosemary Nelson, leading loyalist Billy Wright and Catholic father of two Robert Hamill.
The Finucane family, human rights campaigners and nationalist politicians, as well as Judge Cory, have expressed alarm at moves by the government to ensure the tribunal into Mr Finucane's murder is held under the Inquiries Act, which was passed earlier this year.
They have claimed the Act will suppress the truth about what happened, with Amnesty International saying crucial evidence could be omitted from any final report at the government's discretion.