A parliamentary committee including the former senior judge in England and Wales has warned that giving a Northern Ireland paramilitary watchdog immunity from legal challenge encroaches on a fundamental right.
The Independent Reporting Commission (IRC) is to be established by the British and Irish governments to report on continued republican and loyalist paramilitary activity.
Earlier this year dissident republicans opposed to the peace process killed a prison officer in a bomb attack and police have said they are investigating if an explosive device found in Lisburn on Saturday had fallen from a vehicle.
Immunity is intended to protect the IRC from legal challenge by those it reports on.
Former Lord Chief Justice for England and Wales Lord Judge sits on an influential House of Lords Committee which raised concern, saying : "The right of access to court is a fundamental common law constitutional right.
"The immunity from suit conferred on the commission by Clause 3 clearly represents a major inroad into that right."
Proposals for the commission were part of last year's Fresh Start deal between the Democratic Unionists, Sinn Fein, and the British and Irish governments, which averted a return to direct rule of Northern Ireland from London amid a crisis over the IRA.
Last year Northern Ireland's senior police officer George Hamilton said the Provisional IRA still existed after a murder was linked to its members.
The new IRC will be charged with holding the UK Government, Northern Ireland Executive and Irish government to account.
The Lords committee is chaired by former Conservative Scotland secretary Baron Ian Lang, and it examined the Northern Ireland (Stormont Agreement and Implementation Plan) Bill, which became law in May.
The legislation implements key Government commitments under the Fresh Start Agreement.
Lord Lang wrote: "By extending a very wide immunity subject only to a purely discretionary waiver, the Bill therefore makes possible the exercise of the right of access to court in respect of the commission only in circumstances that the commission is prepared to accept.
"The immunities provided for the commission have implications for the constitutional right of access to court."
Lord Dunlop, spokesman for the Northern Ireland Office (NIO) in the Upper Chamber, said the IRC was to report annually on progress towards ending paramilitary activity connected with Northern Ireland.
"It is considered that the immunities and privileges are necessary to ensure that the IRC is able to fulfil its functions.
"In particular, they are not only directed at the need to secure the independence of the IRC from the sponsoring governments but also to ensure that the body cannot be subject to legal challenge by those who may be the subject of its reports."
The Independent Monitoring Commission (IMC), which considered the paramilitary ceasefires between 2004 and 2011, also enjoyed immunity from legal challenge.