Gerry Adams has accused the Government of undermining the Assembly by breaking promises – including over the controversial issue of 'on-the-runs'.
And the Sinn Fein leader warned that London has acquiesced to the most "reactionary and negative" elements of unionism in a way that threatens the stability of the political process.
But the main focus of his speech to his constituency association last night was the failure of the multi-party negotiations on flags, parading and dealing with the past chaired by US diplomat Dr Richard Haass.
The Louth TD is the most senior republican in recent weeks to indicate the Stormont stalemate could be in danger of developing into a full blown crisis, following warnings from national chairman Declan Kearney and the next Speaker of the Assembly, Mitchel McLaughlin.
Mr Adams said that 16 years on from the signing of the Agreement, the Government had reneged on key deals for republicans.
"Its failure to support the Haass proposals; its decision to unilaterally breach its Weston Park Commitments on resolving the issue of OTRs; and its refusal to implement key outstanding elements of the Good Friday Agreement, is undermining the political structures of the Good Friday Agreement and undermining public confidence," he said.
Rather than the good faith and commitment required to implement agreements, London had broken commitments which was having the effect of "emboldening intransigent unionism", he said.
"In the short term people from working-class communities – mostly young people – are paying the price for the stupidity of the flags protests and the violence which accompanied those protests," he said.
Mr Adams said his party's aim in the coming weeks is to achieve an agreement on the so-called Haass 'final text'.
While the Irish government has agreed it represents the "best way forward" it was now up to London to take up a clear position in support of the proposals, Mr Adams said.