Sinn Fein MLA Máirtín Ó Muelloir has accused DUP leader Arlene Foster of the "same old arrogance" after she called nationalism "narrow and exclusive" during a speech in England.
Mrs Foster gave a wide-ranging speech at a Policy Exchange event titled "The Union and Unionism" in London on Monday morning.
She told the crowd that unionism was inclusive in comparison with nationalism.
"Unionism is at its best when it is confident, outward looking and welcoming. We must move forward with confidence," Mrs Foster said.
"Citizenship and rights are essentially unionist issues. They are issues we should set out to reclaim. Nationalism is by its nature narrow and exclusive. Being a Unionist is the opposite. Unionism stands for pluralism and multi-culturism. We are inclusive and welcome all.
"Confident unionism can capture the diversity that nationalism cannot. It transcends nationalism, and allows individuals to express the cultural values or identity they wish.
The surest way to cement the Union is for Northern Ireland to be open and provide a successful environment in which to live and work. A Northern Ireland which embraces differing cultures and where minorities feel valued - is one that few will choose to abandon."
The DUP leader reitereated her party's commitment to Brexit and stressed that there was no desire for a hard border in Ireland.
"We voted in the referendum as one nation. We must implement that decision and leave as one nation. Again, Northern Ireland is a central element to that debate. But unionists, throughout the country - those who voted remain and leave – accept that we must exit the EU in a way that causes no damage to the economic and constitutional integrity of the United Kingdom, the Fermanagh and South Tyrone MLA said.
"And as a unionist I see no logic or rationale for a hard border being created between Northern Ireland and the Irish Republic. Indeed we do not want to see that at all.
"The only people stirring up myths of border checkpoints are those who are committed to unpicking the Union. They seek to use such imagery to advance and build support for their long-term political objective. They will not succeed."
Describing her speech as a ‘missed opportunity’, South Belfast MLA Ó Muelloir said Arlene Foster had produced ‘no new thinking but more of the same old arrogance’.
“Unfortunately, this much-trumpeted speech from Arlene Foster has failed to provide any indication that the DUP is prepared to end its blockage of the power-sharing institutions," the Sinn Fein MLA said.
“In her speech, Arlene Foster accused nationalism of being ‘narrow and exclusive’ while lauding unionism as standing for ‘pluralism and multi-culturalism’.
“Quite apart from the arrogance of dismissing the entire nationalist population in this way, Arlene Foster’s comments quite simply don’t stack up to the reality.
“Where is the multi-culturalism for Gaelic speakers who are still being denied equal language rights by the DUP?
“Where is the pluralism for the members of our LGBT community who are still being denied marriage equality by the DUP?," Mr Ó Muelloir asked.