The Prime Minister has appealed to politicians in Northern Ireland to think of the "nobler processes and the great noble principles that were put in place in the past" to help the region's crisis-hit political institutions.
Mr Cameron said the creation of devolved power-sharing in Belfast was "one of the most inspiring things that I've seen" and urged politicians to invoke that "spirit".
His comments came as DUP parliamentary group leader Nigel Dodds warned: "We have now reached the tipping point, indeed in my view we have gone beyond the tipping point."
The ministerial Executive at Stormont has been under threat of collapse since police confirmed the IRA was still in existence and that individual members were involved in the murder of a man in Belfast last month.
Crunch talks, scheduled to last for four to six weeks, were convened by the British and Irish governments at Stormont House following the police revelations which rocked the political establishment.
Mr Dodds told MPs: "The Prime Minister will be aware that the situation in Northern Ireland - already grave following the IRA murder in August in Belfast - has escalated to new heights with the arrest today of the chairman of Sinn Fein in connection with that incident, and indeed other leading members of Sinn Fein. We warned about this earlier this week."
Mr Dodds warned of the "very grave state" of the devolution process unless action was taken.
He added: "We have now reached the tipping point, indeed in my view we have gone beyond the tipping point. The Prime Minister is aware the First Minister has met the Secretary of State this morning, he has put a proposal to her.
"Does he now accept that unless he and others take action that we are in a very grave state as far as devolution is concerned? We want to see government but only those committed to exclusively peaceful and democratic means can be in government.
"The people of Northern Ireland cannot be punished, it is Sinn Fein who should be dealt with. Does the Prime Minister agree?"
Responding, Mr Cameron acknowledged that "we are at a very difficult phase" of discussions in Northern Ireland.
He said there was no justification for paramilitary organisations and structures in Northern Ireland or anywhere else, adding they were a "blight on our society, they are not wanted, they should be disbanded on every occasion and on every side".
Mr Cameron said: "The only thing I would appeal to members in the DUP, the UUP, SDLP, the Sinn Fein members who don't take their seats in this House, as someone who sat on those benches and watched while the peace process was put together and the power-sharing arrangements were put in place, it was one of the most inspiring things that I've seen as a human being and a politician, to see politicians put aside their differences, put aside concerns about appalling things that have happened in the past and decide to work together.
"The appeal I would make to all of you is, please, have that spirit in mind, it was an amazing thing you all did in Northern Ireland when you formed that administration and that assembly.
"We'll do everything we can to help you but let us think of the nobler processes and the great noble principles that were put in place in the past, and let's do it again."