A forum of politicians is to call on the British government to release secret files on the Real IRA suspects behind the Omagh bombing.
Nobody has been convicted for the 1998 attack, the biggest atrocity of the Troubles, which killed 29 people.
The issue will be raised by more than a dozen cross-party members of the British-Irish Inter-Parliamentary Body (BIIPB) including Labour MP Andrew Mackinlay and Fine Gael TD Brian Hayes.
Mr Mackinlay will table a motion at the meeting calling on the British government “to disclose immediately to the legal counsel of the families of the victims of the Omagh bombing, details of and all information relating to and arising from, the request made by the RUC to British government Communications Headquarters (GCHQ), for surveillance of those subsequently suspected of preparing and carrying out the atrocity in Omagh a decade ago, including transcripts and timelines”.
Memorial events were held in Omagh in August to mark the 10th anniversary of the attack.
The BIIPB is convening for its two-day meeting in Newcastle on Monday.
Irish and British parliamentarians will also examine proposals to remember the past with an address from Lord Eames and Mr Denis Bradley, co-chairs of the Consultative Group on the Past.
The agenda also includes a briefing by Northern Ireland Secretary Shaun Woodward on recent political developments in Northern Ireland and his assessment of the future.
The body will be attended for the first time by an official Ulster Unionist delegation led by Lord Maginnis of Drumglass, a former MP, and Stormont MLA David McClarty.
“This reverses almost two decades of a Unionist boycott of the Body, which will change its name to the British-Irish Assembly to accommodate Unionist concerns,” said a BIIPB spokesman.
There will also be a presentation from Minister for Communications, Energy and Natural Resources Eamon Ryan on how to maximise energy on the islands of Ireland and Britain and a report on how to boost renewable energy will also be debated.
The body will also hear calls for the creation of a code of conduct for employers of migrant workers “to protect vulnerable communities from exploitation”.
The BIIPB was formally established in 1990 as a link between Westminster and Dublin, with 25 British and 25 Irish members drawn from the upper and lower houses of both parliaments.
In recent years the body has been extended to include representatives from the Welsh Assembly, the Scottish Parliament, the Northern Ireland Assembly and the Isle of Man and Channel Islands.
The joint chairmen of the BIIPB are former Northern Ireland Secretary and Labour MP, Peter Hain and Donegal North-East Fianna Fáil TD Niall Blaney.