The Northern Secretary Dr John Reid has cancelled a meeting between himself Sinn Fein that was set for tomorrow.
Talks set for next week between the party and Prime Minister Tony Blair at Downing Street are expected to go ahead but Sinn Fein president Mr Gerry Adams has reacted angrily to Dr Reid's decision describing it as a "juvenile".
"At a time when there is a real and urgent need to manage the fallout from recent events and when the British Secretary of State has questions to answer, his cancellation of the meeting which we have scheduled tomorrow will further undermine that and republican confidence," Mr Adams said.
"I find his behaviour juvenile and incomprehensible. I do expect to meet the Taoiseach Bertie Ahern in the next day or so and am making arrangements for a meeting with Mr Blair," he said.
Northern Ireland Office sources said the meeting had been called off because the Prime Minister was anxious for discussions with Sinn Fein to get to the bottom of the allegations being levelled against republicans over the infiltration of the British government's headquarters in Belfast.
Earlier, Sinn Fein's Mr Martin McGuinness said he was convinced party official, Mr Dennis Donaldson, who will tomorrow be charged in connection with the police investigation that saw a Sinn Fein office in Stormont raided last week, is innocent.
His comment came as unionists threatened to pull down the power-sharing institutions over allegations that Sinn Fein acquired sensitive Government documents from a mole in the Northern Ireland Office.
Mr McGuinness said: "Allegations have been flying about left, right and centre ... I have many questions to ask about that.
"Was the Speaker of the Northern Ireland Assembly told of the bogus raid on the Sinn Fein office at Stormont because everybody knows that there was nothing found in the Sinn Fein offices?
"Did the First Minister know? Did the Deputy First Minister know?
"At what operational level was the decision to raid our office taken?
"One thing that we are sure of since our office was invaded in Stormont, we were absolutely sure that somebody was going to be charged.
This morning Dr Reid rejected Sinn Fein claims that Friday's raids were a publicity stunt and insisted a "grave crisis" in the peace process had developed.
Sinn Fein claim the raid was a media stunt to undermine the party while the Taoiseach Mr Ahern said the 200 police officers and masses of jeeps used in the raid was "over the top".
Dr Reid said the time had now come for republicans to give an assurance they will abandon all paramilitary activity in a bid to save the peace process.
Dr Reid declared today: "I think we are at a stage which is not only serious, or grave as the Irish Prime Minister said in the last three days. I think this is at a critical stage.
"Somehow we have to have an assurance that if these things have happened in the past, they will happen no longer.
"Now we have never had a signal of that nature. The IRA have never been prepared to say the war is over or that their army is being stood down.
"We have come a huge way in terms of the ceasefire. There have been no attacks on the police or the [British] Army, no planting of bombs, but the constant drip feed of allegations that the maintenance of the apparatus of terror is being carried on is undermining hugely the confidence in this Agreement," Dr Reid told the BBC1 Breakfast with Frost programme.
His call came as unionists threatened to begin moves to collapse the power-sharing executive at Stormont over the latest allegations against republicans.
Ulster Unionist MP Jeffrey Donaldson warned yesterday that if the British government did not act within days to expel Sinn Fein from the Northern Ireland Executive following the police raid on the party's offices at Stormont, his party would do so, in the Assembly.
If that measure were to fail, UUP ministers would pull out of the executive - forcing its collapse.
Dr Reid said that the government was seeking to bring forward a meeting between Prime Minister Tony Blair and Sinn Fein which was scheduled to take place in 11 days.
He denied republican claims that Friday's arrests and raids were politically motivated or that there would be any political interference in the investigation.
"Over the next couple of weeks I think we are in a critical position," he said.
"We certainly need answers from Sinn Fein and the Republican Movement.
"The people of Northern Ireland need answers and certainly the Prime Minister wants answers.
"He had a meeting scheduled for the week after next, not the coming week, and he is trying to bring that forward and meet (Sinn Fein leader) Mr (Gerry) Adams to find those answers.
"I am passionate about this peace process but I cannot keep it going when there is a prima facie case, when these sort of things are happening, coming on top of the Colombia adventure, the burglary in Castlereagh, the charges being brought against an individual for targeting and now this."