The Duke of Edinburgh and the Prime Minister, Tony Blair, will also attend the service, the date and venue for which have yet to be finalised.
A No 10 spokesman said Mr Blair believed that the wishes of the relatives of the victims should be "paramount", and therefore the timing, location and form of the service would be decided in consultation with them.
The news emerged as towns across Ireland - both north and south - were expected to fall quiet today for a minute's silence in memory of the victims.
In Omagh itself, the Irish President, Mary McAleese, will join Northern Ireland's political leaders and up to 20,000 other people in a remembrance service, close to the site of the blast.
In Belfast, St Peter's Roman Catholic cathedral will ring its funeral bell, with the names of the victims read out between each toll.
Elsewhere, the leaders of the four main churches in Northern Ireland, who will all attend the Omagh service, have asked that people observe a silence at 3.10pm - the time at which the bomb exploded last Saturday.
The Irish Prime Minister, Bertie Ahern, who will also attend the service, yesterday urged everyone in Ireland to observe the silence.
Twenty-eight people died in the blast. Of the 78 injured survivors still in hospital, eight are critically ill.
t Two men are still being questioned by the RUC over the blast.
The police yesterday released Shane Mackey, 19, the son of the Omagh councillor Francis Mackey, chairman of the 32 County Sovereign Committee. Two others have already been released.