Five other Central American countries have already ratified the agreement.

Costa Rica is the only country to hold a referendum on the Central American Free Trade Agreement, which will also cover Guatemala, El Salvador, Honduras, Nicaragua and the Dominican Republic.

Costa Ricans have for many years enjoyed more stability and a higher standard of living and public services than their neighbours in Central America.

Earlier Mr Arias said a "No" vote would undermine that position.

"Costa Rica would be saying 'no' to a market of 300 million inhabitants, and that would be a grave error," he said.

The issue has provoked repeated protests in Costa Rica, including a two-day strike by public workers last year.<!-- E BO -->