Speaking during his first appearance at the House of Commons despatch box, Mr Mandelson stressed there was no alternative to the Good Friday accord.

"There is no doubt that if it were to fail that what would follow would be a collapse of the ceasefires and a return to violence," he said.

"As far as the Good Friday Agreement is concerned, it's absolutely essential for that to work, there is no alternative to that agreement, there is no plan B should it fail.

"And if it were to fail, I have no doubt what would follow would be a collapse of the ceasefires and renewed violent confrontation.

Violence a 'daily reality'

He said life was immeasurably different now to what it was like ten years ago when paramilitary organisations were not on ceasefire.

"A decade ago, 80-100 people were being killed each year in Northern Ireland - violence was a daily reality, politics were at a stalemate and there was little or no dialogue between the parties."

The BBC's David Eades: "The leaders are talking, but not budging"

He urged paramilitaries to express in certain terms their unambiguous commitment to exclusively peaceful means.

"The key to peace is persuading republicans and other paramilitaries to engage exclusively in democratic politics and to renounce violence now and forever.

"They have come a long way and they are still travelling in the right direction but many have not yet completed their journey.

Andrew Mackay: Bipartisan approach "with reservations"

Shadow NI secretary Andrew Mackay reiterated the Conservative Party's commitment to a bipartisan approach to Northern Ireland with reservations.

"We just cannot give him a blank cheque unlike the previous paymaster general," said Mr Mackay in an obvious reference to Mr Mandelson's enforced ten-month exile from the Cabinet table.