An investigation is underway into the theft of a union flag which was being flown at a civil service building in Londonderry to mark Queen Camilla’s birthday.
BBC News NI understands an Irish tricolour was erected in its place.
The Department for Communities said it was aware of the “unauthorised” removal of the union flag from its offices in Lisahally on 17 July.
The Police Service of Northern Ireland (PSNI) said it was being treated as a “sectarian hate crime”.
Under legislation, the union flag is flown from government buildings in Northern Ireland on designated days each year.
It was being flown from the civil service building at Lisahally to mark the 78th birthday of Queen Camilla on 17 July.
The department did not confirm that the Irish flag was erected on the flagpole.
But BBC News NI has spoken to workers who say this was the case.
The PSNI said the theft of the union flag was reported to them last Thursday afternoon and they have appealed for anyone with information to come forward.
The Democratic Unionist Party (DUP) described the removal of the union flag and its replacement with the Irish tricolour as “an idiotic act”.
“Such silly attention-seeking behaviour is disrespectful and totally unacceptable,” Foyle MLA Gary Middleton said.
“Those involved should be dealt with by the law.”
Ulster Unionist councillor Darren Guy said those responsible were fuelled by “hate and sectarianism”.
“Over the past few weeks, we have had acts of theft and vandalism occur throughout the city and district,” he said.
“Soldier statues beheaded at a memorial in Artigarvan; a historical US flag stolen from Foyle College; poppy wreaths had to be removed immediately after being laid in remembrance of the Battle of the Somme – the one left was stolen that night and countless attacks on Protestant homes in The Fountain [a mainly unionist estate].”
Guy criticised the SDLP and Sinn Féin over their response to similar incidents.
“Yet, these actions fuelled by hate and sectarianism continue, unchallenged, from their own elected representatives,” he said.
“If this is acceptable now, then it’s not surprising that unionists will never want to be part of the pipe dream that is a united Ireland.”
But the criticism was rejected by Sinn Féin and the SDLP.
Sinn Féin said it had been “unequivocal, consistent and vocal” in asserting that all cultures must be respected.
“All and every incident of hate crime must be called out,” a spokesperson said.
“In stark contrast we have seen the selective approach and double standards of unionist politicians in response to illegal bonfires and hate crimes.”
SDLP MLA Mark H Durkan said the incidents were damaging community relations.
“The SDLP has always called urged tolerance around these issues and condemn those who engage in any attacks, sectarianism or intimidation,” he said.
“I would urge anyone involved in this kind of activity to stop, they are achieving nothing but hurting their own city.”
