Relatives of the Omagh bomb victims will today ask Tony Blair to sanction a cross-border public inquiry into the Real IRA atrocity.
Representatives of the 29 victims and those injured in the August 1998 blast have long campaigned for an audience with Blair.
But Michael Gallagher, whose son was killed in the no-warning attack, said the delay in reaching Downing Street has meant their case is better prepared.
Mr Gallagher said: “The purpose of the meeting is to ask the Prime Minister to grant the families of Omagh a full cross-border public inquiry so that we can bring some degree of closure to the public face of Omagh.
“There is ongoing criminal and civil proceedings but once they are out of the way it is within the Prime Minister’s powers to grant us a full public inquiry.”
Mr Gallagher said he first tried to arrange a meeting with the Prime Minister in 2000.
“It has been a long wait but maybe it is a good thing as we are better prepared than we were five years ago,” he added.
“We are now in our eighth year and we don’t want to go on forever, having more damaging revelations every six months.
“We want all the issues examined in a public forum where it will be clear and transparent.”
Mr Gallagher will be joined in London by three representatives of bereaved families and one spokesperson for the injured.
One man is currently facing charges in connection with the Co Tyrone blast, which resulted in the biggest single loss of life of the Troubles.
But the trial of Sean Hoey, 36, of Molly Road, Jonesborough, Co Armagh is unlikely to go ahead until well into next year.
Last week the families met the Northern Ireland Police Ombudsman Nula O’Loan, whose officers are carrying out an investigation into certain aspects of the police probe.