Amnesty International today branded as "outrageous" the government's failure to hold an independent inquiry into the 1989 murder of the Belfast solicitor Patrick Finucane.

Another human rights organisation, British-Irish Rights Watch, which monitors allegations of collusion in Northern Ireland, described the lack of a public hearing into one of the most controversial killings of the Troubles as a significant "gap in the peace process".

Their criticism follows the release this week of US diplomatic cables, obtained by the WikiLeaks website and published by the Guardian, which revealed that MI5 offered to hand over its files to any forthcoming inquiry.

Finucane, a prominent civil rights and defence lawyer, was shot dead in front of his wife and three children by loyalist gunmen working with British security forces.

The security service's offer to share its files was revealed in a cable from June 2005, written by the US ambassador to Dublin, James C Kenny, which reported on a meeting between the head of MI5 and Mitchell Reiss, the US special envoy to Northern Ireland.

In an account of the meeting between Reiss and Ahern, the ambassador wrote: "Reiss briefed him on his talks in London, including with the head of MI5 [Eliza Manningham-Buller], who committed to turning over all evidence her agency has to the inquiry but she was adamant that the inquiry will proceed using the new legislation."

Amnesty said the fresh information strengthens the case for a public investigation. Its UK director, Kate Allen, said: "The UK government's failure to hold a properly independent inquiry into the killing of a prominent lawyer in this country is outrageous and with each day that passes and each new revelation that comes to light, that failure becomes yet more outrageous.

"These new revelations show concerns that elements within the UK security establishment have sought – to date, successfully– to avoid an inquiry and, further, that MI5 holds important files on the killing that have yet to be disclosed."

She added: "Over 20 years on from the murder of Patrick Finucane, the truth about his killing is still being kept from his family and from the public. The UK government should establish an independent public inquiry without any further delay."

Jane Winter, the director of British-Irish Rights Watch, said: "We welcome the publication of these documents by WikiLeaks and the Guardian. They confirm what we always knew, which is that international opinion, whether from Ireland or America, has long been of the view that Patrick Finucane's murder deserves an independent inquiry, and that so long as it remains unresolved there will be a gap in the peace process."

In July 2003, the European court of human rights ruled that "the proceedings following the death of Patrick Finucane failed to provide a prompt and effective investigation into the allegations of collusion by security personnel", and that there had therefore been a violation of article two of the European convention on human rights in his case.

Sir John Stevens, the former Metropolitan police commissioner, who carried out three inquiries into allegations of collusion between the security forces and loyalist paramilitaries in Northern Ireland, has stated that his investigations uncovered evidence of "collusion, the wilful failure to keep records, the absence of accountability, the withholding of intelligence and evidence, and the extreme of agents being involved in murder".

The full findings of his investigations remain secret – they have not even been seen by the Finucane family and their lawyers.