The motion, which will be brought before the Assembly chamber by Alliance leader David Ford on Tuesday, will call for "the establishment of a formal, cross-border, legally binding process designed to secure full disclosure from the Intelligence Services and Security forces in the United Kingdom and the Republic of Ireland."

In the most fatal act of violence during the Troubles, 29 people and unborn twins lost their lives in August 1998.

Mr Ford says the victims' families have a right to know the truth.

"Clarity and a full account of events are the least that those affected by the bomb deserve," he said.

"People's lives were ripped apart by this bombing and victims continue to suffer because of all the unanswered questions. Given the cross-border aspects of the crime, many will not gain closure without a full cross-border investigation process.

"This process simply has to be legally-binding to ensure that no details can be withheld. People deserve the whole truth, not some politically expedient report on events," he added.

The motion has been welcomed by Michael Gallagher, Chairman of the Omagh support and self-help group.