Spain's prime minister says a fast-track constitutional amendment to enshrine budgetary discipline is essential for the country, although he understands some people are griping at the quick pace of the change.

Jose Luis Rodriguez Zapatero announced the proposed amendment last week after his ruling Socialists reached agreement with their archrivals, the conservative Popular Party. It could be approved as early as next week.

Labor unions are furious over the idea of a deficit cap and want a referendum. Regional governments are angry because the reform will severely trim their financial autonomy.

But Zapatero said Wednesday Spain needs the change now to guarantee the stability and credibility of state finances.