THE Alliance, Democratic Left and the Workers' Party have welcomed the SDLP's decision to, contest the May election and to consider taking part in the forum.
However, Republican Sinn Fein and the INLA's political wing, the Irish Republican Socialist Party, were highly critical of the announcement.
The Alliance Party's general secretary, Mr David Ford, said: "Alliance is looking forward to working with all democratic parties in an inclusive process."
Mr Seamus Lynch, of Democratic Left, described the SDLP's response as constructive. "While there are a lot of misunderstandings and misgivings among the electorate about this whole process, the only place for democrats to be is in a discussion," he said.
People did not want their political representatives to adopt an abstentionist policy, but to "get in there and make the best of it", he added.
The Workers' Party's Northern chairman, Mr Tom French, also welcomed the SDLP's announcement. "The new Northern Ireland forum is an essential element of the search for permanent peace and new political structures," he said.
The president of Republican Sinn Fein, Mr Ruairi O Bradaigh, accused the SDLP of surrendering to a British unionist scheme for "an updated Stormont with cross Border boards". He added: "It took 50 years to get rid of the old Stormont. Why promote its return?
Mr Kevin McQuillan, of the IRSP condemned the SDLP and Sinn Fein for deciding to contest the election. He predicted that both parties would take part in the forum.
The IRSP has been excluded from the British government's list of parties approved to contest the election. Mr McQuillan said that the SDLP and Sinn Fein had become British approved parties" and were supporting an "anti democratic election" and a unionist agenda.
Mr McQuillan was particularly critical of Sinn Fein, which he said had failed to take a principled republic opposition.
"The IRSP will not accept the result of elections we have no part in," he added. "We will not accept a settlement arising from talks into which we have had no input."
. A Government spokesman in Dublin responded in a low key manner to the announcement of SDLP and Sinn Fein participation in the election. The whole point, he said, was to get to all party talks on June 10th, and in that sense the decision was welcome. He hoped that all parties would be at the negotiations.
Ms Mary Harney, leader of the Progressive Democrats, also welcomed the participation of the SDLP and Sinn Fein in the elections.