Political leaders from four Northern Ireland parties have written to the secretary of state urging him to reconsider his position on a public inquiry into the murder of Pat Finucane.
The family of the Belfast solicitor was left furious on Monday after the Government said no to an immediate inquiry.
The letter was signed by Sinn Fein's Michelle O'Neill, the SDLP's Colum Eastwood, Alliance deputy leader Stephen Farry and the Green Party's Clare Bailey.
They told Brandon Lewis the Government's decision was "neither a credible nor tenable position".
The latter adds: "It was an insult to the Finucane family. Your approach to this matter is now a matter of serious public concern.
"To avoid a public inquiry, you have clearly made a calculated decision to embark on a high-risk distraction strategy that now places both the Office of the Police Ombudsman and the PSNI in the midst of a historic murder investigation at a time when the intent is to take legacy out of policing.
"Your suggested approach potentially risks wider confidence in the rule of law and the administration of justice."
Pat Finucane, 39, who represented republican and loyalist paramilitaries during the conflict, was shot dead in his family home in north Belfast in February 1989 by the UDA in an attack found to have involved collusion with the state.
Mr Lewis said he was not taking a public inquiry off the table, but said further examinations of the case by police and a police watchdog should conclude first.
The Finucane family are seeking the support of President-Elect Joe Biden for their ongoing campaign for a public inquiry.
The Irish Government and 24 members of US congress were among those urging the UK Government to order an inquiry ahead of Monday's announcement
Mr Finucane's son John, a solicitor and Sinn Fein MP for north Belfast, said Mr Biden had previously voiced support for his family's campaign.
"When he was then Senator Biden as chair of the Senate Foreign Relations Committee, he supported our calls for an inquiry," he told the BBC.
"I appreciate he's going to be in arguably the busiest role on the planet, so we aren't we aren't blind to that, but President-Elect Biden has already shown a clear interest in Irish affairs, he has history in supporting our campaign.
"We will be seeking his fresh support on this matter and I think that we can enjoy a degree of confidence that given the bipartisan way in which we have received support from America this is something that may well enjoy his future support."
Mr Finucane added: "As my mum said very well yesterday: our campaign will continue."
WATCH: Geraldine Finucane speaks to UTV Live
Explaining why the Government had decided against a public inquiry at this stage, Mr Lewis told Parliament outstanding issues before the Police Ombudsman's office concerning original police investigations should be addressed first.
He said there could also be a review of the case by police officers at the start of next year, potentially carried out by a force from outside Northern Ireland.
Following Monday's announcement, the chief constable of the PSNI, Simon Byrne, made clear there were no new lines of inquiry in the case.
Mr Finucane's widow Geraldine and the couple's three children have been campaigning for decades for a public inquiry to establish the extent of security force involvement.
Last year, the Supreme Court said all previous examinations of the death had not been compliant with human rights standards.
The Supreme Court acknowledged Mrs Finucane had been given an "unequivocal undertaking" by the Government following the 2001 Weston Park agreement with the Irish Government that there would be a public inquiry into the murder.
However, the judges found that the Government had been justified in later deciding against holding one.
The court said it was up to the Government to decide what form of investigation was now required, if one was feasible.
Amid an almost two-year Government delay in responding to the judgment, Mrs Finucane took fresh judicial review proceedings against the state.
Monday's announcement by Mr Lewis represented the Government's formal response to the Supreme Court's ruling.