The leader of the Democratic Unionist Party (DUP) has called for Irish President Michael D Higgins to reconsider his decision to turn down an invite to a church service to mark Northern Ireland's centenary.
Sir Jeffrey Donaldson said "politics are at play" in the decision.
Earlier, the Alliance Party also said President Higgins should reconsider.
The Queen has been invited to the service, which has been organised by Christian church leaders.
It will take place at the Church of Ireland cathedral in Armagh city on 21 October.
The inter-denominational service is part of a programme of events to mark the centenary of the partition of Ireland and the formation of Northern Ireland in 1921.
On Wednesday, a spokesperson for President Higgins said he was "not in a position to attend the ceremony and this has been communicated to the organisers".
It is understood he had been expected to attend. He did not offer any further explanation of his decision.
RTÉ reports that the Irish government said it did not advise President Higgins to decline an invitation to attend the church service.
But the Irish government told BBC News NI that "the president's diary is a matter for the president and it would not be appropriate to comment".
"Any invitation for a government representative to attend the ceremony will be considered," it added.
Sir Jeffrey, who has written to President Higgins about the matter, said it was a "disappointing and retrograde step" by the president.
"We hope he will rethink this decision - it is not conducive to the kind of respectful relationship with the head of state of our neighbouring jurisdiction," he said.
"In the absence of clarity around the reason for him not attending many may reach a conclusion that it is a decision based on politics."
Sir Jeffrey said the service was about "marking the centenary", which he added was a "fairly neutral term".
Former Presbyterian Moderator Dr John Dunlop said he was disappointed by the decision.
"This is a time when I think you have to consolidate bridges especially in a time like this when things between the Republic of Ireland and Northern Ireland or between Ireland and Northern Ireland are a bit fraught and a bit fragile," he said.
'Surprising and uncharacteristic'
Alliance Party MP Stephen Farry also said President Higgins should reconsider his position.
"This event is very much in keeping with the reconciliation efforts of successive Irish presidents and the Queen over recent years and the inclusive approach to marking the decade of centenaries," he said.
However independent assembly member Trevor Lunn, formerly of the Alliance Party, tweeted in support of the president's decision.
President Higgins has been a good friend to Northern Ireland . I wouldn’t criticise his decision not to join in a commemoration of the partition of his country.— Trevor Lunn (@TrevorLunnLV) September 15, 2021
I don’t see it as a discourtesy to Her Majesty, just a correct if difficult decision.
"I don't see it as a discourtesy to Her Majesty, just a correct if difficult decision," he wrote.
SDLP leader Colum Eastwood said people should take President Higgins "at his word when he says he can not be there".
Mr Eastwood said President Higgins had a track record of reconciliation.
He said: "President Higgins' term has been marked by diversity and reaching out to all traditions across our island.
"He has been an advocate of bringing people together and I think people should not read too much into this given all that he has done for community relations and reconciliation across Ireland."
Ulster Unionist Party assembly member Mike Nesbitt described the move as "surprising and uncharacteristic from someone who has shown a consistent willingness to reach out and promote reconciliation".
"The reciprocal state visits were a high watermark in Anglo-Irish relations and until we know the reason why he is not in a position to attend, we cannot be critical," he said.
The leader of Aontú, Peadar Tóibín, said President Higgins had made "the right decision not to attend the event".
"There isn't a president in the world that would attend an event to mark, commemorate or celebrate the partition of his or her country," he said.
The Independent Group of Senators in the Republic of Ireland has expressed its concern at President Higgins' decision.
Senator Gerard Craughwell, who is a member of the group, said he was "deeply disappointed".
"It is a commemoration service, not a celebration service, so there should be no political connotations whatsoever to the event," he told BBC News NI.
So far the church leaders have made no comment about the issue.
President Higgins is in Italy and has not yet responded to calls for him to reconsider his decision or give his reasons.