Mike Nesbitt has urged the Prime Minister not to dissolve Stormont because of the row over welfare reform.
The Ulster Unionist Party leader discussed the issue with David Cameron on Monday, in their first meeting since the general election.
With Stormont consensus on welfare reform still a long way off, the political instability in Northern Ireland was at the forefront of Mr Nesbitt's mind during the meeting.
Mr Nesbitt said Mr Cameron was "clearly very concerned" about the lack of implementation surrounding the Stormont House Agreement. The unionist leader said he impressed on Mr Cameron the need not to give in to Sinn Féin.
"My plea to the Prime Minister is not to collapse the institutions because it seems to me that that may be exactly what Sinn Féin would like, with their focus now almost exclusively on the next general eelction in the Republic of Ireland," he told the BBC.
"Martin McGuinness, despite the fact that he's a joint First Minister with responsibility for all the people of Northern Ireland, appears to be allowing himself to be wagged by the tail of Sinn Fein's southern command."
Mr Nesbitt added: "It appears the people of Northern Ireland are suffering to the tune of £2m a week, and there's no real prospect of that ending or being resolved soon. It would be wrong for the Prime Minister to offer Sinn Féin a solution by stepping in and suspending the institutions."