The head of a proposed new commission to deal with Troubles cases says if the UK's controversial plan is found to be contrary to international human rights law then there will "have to be a rethink".
Declan Morgan is the Chief Commissioner designate of the Independent Commission for Reconciliation and Information Recovery.
It is part of a UK government plan to draw a line under all Troubles investigations and is expected to become law in the coming weeks.
The plans have been condemned by victims' groups and political parties in Northern Ireland, by the Irish Government, the Council of Europe and US politicians.
They back a return to previously agreed legacy proposals which leave open the courts as one way for families to achieve justice.
Under the proposal all legacy cases, civil and criminal, along with inquest and other investigations, would cease next May and be passed to the new commission.
If asked, the commission will seek to establish what facts it can about any of the 1,200 unsolved murders with killers being offered amnesty in return for co-operation.
It will then offer reports to families.
Mr Morgan told RTÉ News he expected immediate legal challenges to the compatibility of the amnesty clause with the UK's commitments to the European Convention of Human Rights.
He said if a Belfast court found the amnesty was compatible then the commission would apply it.
If the clause was found to be incompatible then changes would have to be made to the legacy arrangements.
He said once that immunity question was resolved it would mean people could have confidence "that you have a system that is designed to deliver, that is compatible with the Good Friday Agreement and is also compatible with the European Convention on Human Rights".
"You've got to have principles to underpin a commission of this sort and those for me are the three grounding principles upon which we have to base our activities," he said.
Asked what would happen if the courts found against the UK government on the issue of immunity from prosecution, he said: "Then everybody will have to have a rethink."
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A government spokesperson reiterated its opposition to the UK's unilateral legacy plan: "The Tánaiste has raised the Government's concerns around this legislation with British counterparts on multiple occasions.
"The Government believes that the Bill, which is a unilateral departure from the Stormont House Agreement, is not fit for purpose, and raises fundamental issues around compatibility with international human rights obligations.
"We urge the British Government to pause the legislation, and return to an agreed approach that enjoys the support of all major stakeholders, puts victims at its centre and is compliant with international human rights standards."
The controversial UK bill, which will establish the Independent Commission for Reconciliation and Information Recovery, is in its closing stages at Westminster.
It is expected to be passed into law in shortly after the British Parliament returns from summer recess next week.
Opponents of the plan have claimed that the real purpose is to ensure an amnesty for British soldiers involved in controversial incidents during the Troubles and to prevent further embarrassment to the state from damaging revelations during legacy cases.
Mr Morgan said he would have powers to compel people at the highest level to provide him with information, he had reiterated his intention to do so with the British government and he was determined to set out the facts - no matter how controversial - in any case he was asked to review.
