Sinn Fein has accused Mr Peter Mandelson of being "a peace wrecker, rather than a peacemaker". The party has also described the Northern Secretary as a political "disaster".
The comments were made by the Sinn Fein Education Minister, Mr Martin McGuinness, who is in Australia. A Northern Ireland Office spokesman said Mr Mandelson did not wish to respond to the remarks.
Speaking before meeting the Australian Foreign Minister in Canberra yesterday, Mr McGuinness called for a "new approach" by the British government to resolve the current political crisis.
In one of the most outspoken attacks yet on Mr Mandelson by Sinn Fein, he said: "We need a new approach on the part of the British government if the present crisis is to be resolved.
"I very much doubt whether Peter Mandelson has the integrity to recognise this, much less acknowledge it. The unfortunate reality that we must now face is that in Peter Mandelson we have a peace wrecker rather than a peacemaker.
"Peter Mandelson inherited a unique and highly developed opportunity to resolve the historic conflict in Ireland. By his mishandling of this opportunity he has retarded the search for a lasting peace in Ireland and endangered the Good Friday agreement which so many worked so hard to achieve."
Mr McGuinness accused the Northern Secretary of endangering the Belfast Agreement by reneging on British government commitments on demilitarisation and on fully implementing the Patten report on policing.
The Sinn Fein minister said the Provisional IRA initiative on decommissioning had occurred in the context of these promises. "Peter Mandelson has obstructed the delivery of the commitments made by his Prime Minister.
"The IRA initiative was situated in this context and was wholly dependent on these commitments being honoured. The British government reneged. I believe that this was primarily the responsibility of Peter Mandelson.
"And not only has he not honoured commitments given to nationalists and republicans, he has pursued with enthusiasm a pro-unionist agenda ever since opposing demilitarisation, undermining the Patten recommendations and taking power to fly the Union flag over the departments of nationalist and republican ministers. In short, Peter Mandelson is a disaster."