Unionist parties have walked out of talks aimed at resolving outstanding peace process issues, following this morning's decision to bar an Orange Order parade from marching past Ardoyne.
In a joint statement issued by DUP leader Peter Robinson, UUP leader Mike Nesbitt, TUV leader Jim Allister, PUP leader Billy Hutchinson and Ian McLaughlin of the Ulster Political Research Group, the parties said the Parades Commission "place no value on a relationship with unionism and have treated our advice with contempt."
They added: "The DUP and UUP will end their participation in the now fruitless leaders' talks."
The discussions began yesterday and came six months after the Haass talks to resolve the same contentious issues ended without agreement.
Today's statement called for those who opposed the Parades Commission's decision to remain "peaceful and calm."
The statement said: "There will be a graduated unionist response involving the Orange Institution, the PUL community and political unionism. Unionist leaders are willing to share the strain within the political process. Political action in tandem with peaceful and lawful protests is the path we must follow."
The Parades Commission cited the potential for public disorder and negative impact on community relations among its reasons for preventing the July 12 parade proceeding along the Crumlin Road, which is adjacent to the nationalist Ardoyne neighbourhood.
When the same parade was banned last year, rioting occurred in the nearby unionist community in Woodvale. Loyalists have manned a protest camp at the community interface ever since, requiring a policing operation costing almost £10m.
Sinn Fein Deputy First Minister Martin McGuinness said he hoped the statement was not a veiled threat to collapse the Stormont institutions.
"I have just read the combined unionist statement and I don't know what that means - 'graduated response'," he said.
"We will have to wait for someone to articulate what that means. I hope it doesn't mean a threat to the institutions.
"I have made it clear that these institutions have provided very important stability."
He added: "I tell it as it is. I think there is an awful lot of scepticism and indeed cynicism out there in regard to the willingness of unionist parties and the Orange Order to come to an agreement."
Mr McGuinness called for calm in communities.
He said: "I am saying to everyone in north Belfast and further afield, this is time for cool heads, this is a time for people to be calm, this is time for people to recognise that damaging consequences over the course of the next short while, if people resort to violence, will be very, very damaging for all of us, but I think particularly damaging for the Orange Order themselves."
Alliance party leader David Ford launched a blistering attack on the unionist parties.
"I am horrified and disgusted by the behaviour of the DUP and Ulster Unionists," he said.
"They talked about the importance of these talks to resolve the outstanding issues that plague our community and, on a whim today, absolutely unrelated to the business in the talks, they have walked out.
"In doing so they have left us in further problems around parading, as they are raising tensions inevitably on the streets, they have left us with the toxic issues of flags still ahead of us, they have failed to show the kind of joined-up work that we need to see if we are going to build a successful economy and successful society in Northern Ireland and, most of all, they have let down the victims whom they claim to be speaking for."
SDLP leader Dr Alasdair McDonnell said he was disappointed by the unionist response and also called for calm.
"There is now a desperate need for people to keep calm and provide leadership in our communities. Being difficult and being angry is not going to help anyone," he said.
"The SDLP wants to make sure that the next two weeks pass off peacefully in a way that does not damage our city or our people.
"All of the parties need to get back in the room and talk it out; this is the only way of making progress.
"We were making some progress and the SDLP are convinced that there are potential solutions to all issues on the table. One specific parade or one specific issue should not be allowed to derail the entire process that has the potential to resolve the outstanding challenges we face.
"This is a time for calm heads. If we have learned anything in 40 years it is that you can make all the threats you like, you can destabilise what you like but at the end of the day you have to come back to the institutions. We need the institutions to hold our community together.
"The public are way ahead of some of the politicians. They want to see the talks make progress and move into prosperity - that is what the public are looking for from us as politicians and our institutions."
Northern Ireland Secretary Theresa Villiers said the last thing the region needed was another outbreak of public disorder.
"The Parades Commission ruling and subsequent political developments today will provoke strong feelings from many in Northern Ireland but it is essential that the rule of law is respected in any reaction to what has happened," she said.
"The last thing Northern Ireland needs is any kind of public disorder which could put police officers at risk of injury or worse and which would damage Northern Ireland's reputation abroad and undermine efforts to attract jobs and investment.
"Any reaction or protest needs to be both peaceful and lawful, as called for by unionist leaders in their statement today.
"The Parades Commission is the lawfully constituted authority to make decisions on parades in Northern Ireland and their determinations have the full force of law.
"It is very disappointing that the DUP and the UUP do not currently feel that they can continue to take part in the party leader talks at Stormont.
"Ultimately, the only way to make lasting progress on resolving the contentious issues of flags, parading and the past has to be through renewed dialogue between Northern Ireland's political leadership. The Government will continue to do all we can to encourage and early resumption of these talks."
Speaking earlier this week, First Minister Mr Robinson claimed recent decisions to restrict Orange marches taken by the Government-appointed Parades Commission adjudication body was having a destabilising effect.
However Deputy First Minister Mr McGuinness claimed unionists were effectively threatening the institutions at Stormont if the Orange Order and hardline loyalists did not get their way on issues like parades.
Mr Robinson branded the commission a "shambles".
In particular he referred to a determination in Portadown last month that initially permitted a parade only for the commission to change its stance within days.
Mr Robinson said: "A real effort has to be made to ensure that we don't have violence on the streets.
"I abhor violence on the streets and the cause of violence as well, because very often the cause is stupid decisions that are made by bodies that are appointed to make sensible decisions.
"You cannot have a body that takes a decision one day to allow a parade to go down a road, changes its mind the next day and says it can't go down a road, changes their minds to say they'll have a review, changes it again to say they won't have a review.
"What impact does that have on a community? It's an absolute shambles."
He said the unionist leadership would "channel that anger" in a way that was "peaceful and political". Mr Robinson said if the Parades Commission continued to act in a "foolish" way it should be replaced.