Northern Ireland's leaders will meet with Gordon Brown and Irish taoiseach Brian Cowen in Belfast later as the final attempts to save the province's powersharing arrangement get underway.
It follows last-ditch talks between Democratic Unionist party (DUP) leader Peter Robinson and Sinn Fein's Martin McGuinness, which began earlier today.
Negotiations over the devolution of policing and justice powers, as well as the management of contentious parades, have failed to result in a deal so far.
The situation is now becoming critical for Stormont's government. Sinn Fein leader Gerry Adams has described the meetings as "defining and critical" as his party claimed it had done all it could to placate the DUP.
"It is very important that we get a successful outcome to these discussions and we believe that our going there should help bring a conclusion to the devolution issues," Mr Cowen said in No 10.
If Mr McGuinness resigns the move would force Mr Robinson to stand down, given the joint nature of the post. This would mean the demise of the Stormont executive.
"The hope is a solution will be found," Mr Brown's spokesman said this afternoon. "It's a complex situation but he thinks that there is a chance of progress."
The prime minister and Mr Cowen made the "final decision" to travel to Belfast when they met in Downing Street this afternoon, she added.
It is not yet clear whether the prime minister will remain in Belfast overnight. But tomorrow morning's Cabinet meeting has been shifted to Thursday as a "sensible measure", the spokesperson said.
Mr Robinson has temporarily stepped down as first minister for six weeks after the fallout from his wife's affair with a 19-year-old. But he is continuing his role negotiating on policing and justice issues.
"We remain at the table ready and willing to discuss those issues and move the process forward," DUP MP Jeffrey Donaldson said earlier. Mr McGuinness called on the DUP to "make peace" with Sinn Fein, however.
"It has been my life's work over the course of recent times because I passionately believe in power-sharing, I passionately believe in the all-Irish constitution," he said before meetings began today.
"I'm still determined to make things work. It's about delivering power for our entire community, not for sections of our community. It's about working together."