George W Bush: "We wish you well in pursuing justice"

Twenty-nine men, women and children died and hundreds were injured when the Real IRA detonated a car bomb in the County Tyrone town on 15 August 1998 - a Saturday when the centre was full of shoppers.

Mr Gallagher received a letter from President Bush's Northern Ireland advisor Richard Haass, expressing their support for the civil action.

"We are heartened that people in Northern Ireland are turning away from violence and instead relying on normal political and legal means to resolve issues and disputes," Mr Haass said in the letter.

"In that vein, we commend your decision to pursue legal civic actions against those responsible for the Omagh atrocity.

"The president and I thank you for keeping us informed of your endeavours and wish you well in pursuing justice for the victims of the Omagh tragedy and their families."

'Important boost'

Mr Gallagher said the US administration's policy of zero terrorism should be seen as an example.

He told BBC Radio Ulster: "The importance of this is that America will support victims when they stand up to be counted.

"We saw this early on in this administration, when one of the first things they did was to put the Real IRA and the 32 County Sovereignty Movement and the Irish Prisoners' Welfare Association on their foreign terrorists list.

"I think we have seen right through this administration, that when people masquerade as victims supporters or politicians, that they will be challenged, and I think that is important."

The families' campaign has recently been backed by celebrities and politicians including rockstar Bob Geldof, former boxer Barry McGuigan and Northern Ireland Secretary Peter Mandelson.