Prime Minister Boris Johnson is due to attend a church service in Armagh on Thursday to mark the centenary of Northern Ireland's formation.
Irish government foreign affairs minister Simon Coveney and chief whip Jack Chambers will also join the event at St Patrick's Church of Ireland Cathedral.
The Queen had been due to attend but her two-day trip to Northern Ireland was cancelled after she "reluctantly accepted medical advice to rest for the next few days", Buckingham Palace said.
Irish president Michael D Higgins will also be absent from the prayer service, which has been organised by the four main churches on the island of Ireland.
His decision to decline an invitation became the centre of a diplomatic row last month, with Mr Higgins saying he believed the event was not politically neutral.
First Minister Paul Givan, DUP leader Sir Jeffrey Donaldson, Ulster Unionist leader Doug Beattie, Alliance leader Naomi Long and SDLP leader Colum Eastwood are also expected to be present.
Sinn Féin has said it would not accept an invitation to the event, with Junior Minister Declan Kearney saying Mr Higgins was "correct" to avoid it.
The service is to be broadcast live on BBC and RTÉ.
It has been described as a service of reflection and hope, to mark the centenary of the partition of Ireland and the formation of Northern Ireland.
The sermon will be given by Rev Dr Sahr Yambasu, president of the Methodist Church in Ireland - the first African-born leader of one of Ireland's main churches.
Catholic and Protestant church leaders last month said they had been saddened by the "polarised political commentary" around the service.
