Sinn Fein and the Democratic Unionists must not fail to agree a budget, Tom Elliott has warned.
The Ulster Unionist leader accused the power-sharing partners of bad government in a hard-hitting address at his party conference in Belfast.
Sinn Fein has criticised First Minister Peter Robinson for apparently going public on private budget negotiations. Ministers have to agree multimillion-pound reductions in public spending as the economic slowdown bites.
Mr Elliott said: "What we have today is bad government. It's supposed to be co-operative, consensual, power-sharing government. It's not. It's a carve-up - it's Sinn Fein/DUP failure."
He added that there was no agreement on a budget to tackle the economic crisis.
A row threatened to break out in the Executive after DUP leader Mr Robinson used a major speech to press for early agreement and to detail the likely contents of a deal.
But Sinn Fein Regional Development Minister Conor Murphy said he was surprised by the move and added that his party would not be rushed into a deal. He pledged that Sinn Fein was prepared to work through the Christmas holidays.
The clash came after Mr Robinson told a meeting of the Chief Executives Forum in Templepatrick that ministers were nearing agreement on a budget which could freeze Civil Service pay, reduce the use of consultants and quangos, and see ministers take a 5%-10% pay cut.
Mr Elliott, a Co Fermanagh farmer and Northern Ireland Assembly member for Fermanagh and South Tyrone, said there needed to be change at the heart of the government and greater accountability individually and collectively.
He warned the Assembly needed a voluntary coalition with a cross-community basis and an opposition.